Inflammatory bowel disease

Inflammatory Bowel Disease Main page
Patient Information
Overview
Pathophysiology and Causes
Classification Crohn's disease Ulcerative colitis
Differential Diagnosis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2] ; Omar Soliman, M.D.[3] Synonyms and keywords: IBD ; Template:Infobox medical condition

IBD encompasses three clinical entities: Crohn disease (CD), which may involve any segment of the gastrointestinal tract from mouth to anus; ulcerative colitis (UC), which is confined to the colon and follows a continuous pattern of inflammation beginning in the rectum; and IBD-unclassified (IBDU), reserved for cases that cannot be definitively classified as either.

Detailed classification, disease behavior, and extent are addressed in the Classification section below

Overview

Inflammatory bowel disease (IBD) is a chronic, immune-mediated inflammatory disorder of the gastrointestinal tract that follows a relapsing and remitting course, causing significant morbidity and substantially impacting quality of life. It encompasses two major disease entities — Crohn disease (CD) and ulcerative colitis (UC) — as well as IBD-unclassified (IBDU), a category reserved for cases that cannot be definitively classified. While Crohn disease may involve any segment of the gastrointestinal tract from mouth to anus, ulcerative colitis is confined to the colon, beginning in the rectum and extending proximally in a continuous pattern.

IBD is no longer a disease of the Western world alone. Though historically concentrated in North America and Europe, its incidence is rising rapidly across Asia, Latin America, and the Middle East, driven by industrialization and shifting environmental exposures. Globally, an estimated 6.8 million people are affected, and that number continues to grow.

Beyond the gut, IBD is a systemic disease. Extraintestinal manifestations involving the joints, eyes, skin, and hepatobiliary system affect a substantial proportion of patients and may precede intestinal symptoms entirely. The diagnostic and therapeutic complexity of IBD requires a multidisciplinary approach, integrating clinical assessment, endoscopy, biomarkers, and increasingly targeted biological therapies.

Classification

Inflammatory bowel disease (IBD) comprises three clinical entities: Crohn's disease (CD), ulcerative colitis (UC), and IBD-unclassified (IBD-U). CD and UC are further categorized using standardized classification systems that describe age at diagnosis, disease location, disease behavior, extent, and severity. These classifications have prognostic value and help structure treatment selection and longitudinal monitoring. Formal diagnostic testing, detailed endoscopic scoring, and treatment recommendations are addressed in their respective microchapters.

Microscopic colitis (MC), including collagenous and lymphocytic colitis, is a histologically defined inflammatory colitis that is clinically related to IBD but is generally considered distinct from CD and UC.

IBD-unclassified

IBD-U is used when the clinical, endoscopic, radiologic, and/or histologic features do not permit confident classification as CD or UC. The term is preferred for the clinical setting over the older term "indeterminate colitis," which is conventionally reserved for colectomy/pathology specimens that cannot be classified as CD or UC.

IBD-U accounts for approximately 10% of adult IBD and is more frequent in pediatric IBD. Pediatric IBD-U is often dynamic: a substantial proportion of children are subsequently reclassified as CD or UC as additional clinical, endoscopic, and histologic information becomes available.

Crohn's disease: Montreal classification

The Montreal classification categorizes CD according to age at diagnosis (A), location (L), and behavior (B). The suffix p denotes concomitant perianal disease. Location is generally more stable over time, whereas disease behavior may evolve from inflammatory to stricturing and/or penetrating disease.

Axis	Category	Definition / clinical meaning
Age at diagnosis (A)	A1	≤16 years
	A2	17–40 years
	A3	>40 years
Location (L)	L1	Terminal ileum
	L2	Colon
	L3	Ileocolonic
	L4	Upper gastrointestinal disease; used as a modifier of L1–L3 when disease is present proximal to the terminal ileum
Behavior (B)	B1	Non-stricturing, non-penetrating (inflammatory)
	B2	Stricturing
	B3	Penetrating
Perianal modifier	p	Concomitant perianal disease

The L4 designation is particularly important because upper gastrointestinal involvement may coexist with ileal, colonic, or ileocolonic disease. The classification should therefore capture the complete distribution rather than forcing a single anatomic site.

Ulcerative colitis: Montreal extent classification

UC is classified according to the maximum anatomic extent of colonic inflammation. The Montreal system divides UC into three extent categories.

Montreal category	Extent	Anatomic definition
E1	Proctitis	Disease limited to the rectum, distal to the rectosigmoid junction and generally within approximately 18 cm of the anal verge
E2	Left-sided colitis	Disease extending proximally but not beyond the splenic flexure
E3	Extensive colitis / pancolitis	Disease extending proximal to the splenic flexure

Extent is clinically relevant for prognosis, surveillance, and treatment planning, but extent alone should not be used to exclude otherwise appropriate therapeutic options.

Disease severity in ulcerative colitis

No single severity instrument captures every clinically relevant dimension of UC. Validated instruments are used for different purposes, including bedside assessment, clinical trials, endoscopic assessment, and pediatric disease activity assessment.

Tool / framework	Main use	Major components
Truelove-Witts	Bedside assessment of acute severity	Stool frequency, rectal bleeding, pulse, temperature, hemoglobin, and inflammatory markers
Mayo score / partial Mayo	Clinical assessment and clinical trials	Stool frequency, rectal bleeding, endoscopic activity, and physician global assessment; partial Mayo omits endoscopy
Mayo Endoscopic Score (MES)	Endoscopic assessment	Endoscopic severity of mucosal inflammation
UCEIS	Endoscopic assessment	Endoscopic vascular pattern, bleeding, and erosions/ulceration
SCCAI	Clinical assessment	Symptom-based activity assessment, particularly useful in clinical settings
PUCAI	Pediatric disease activity	Pediatric symptom-based assessment

The modern approach to UC severity is multidimensional rather than dependent on a single score. Clinical symptoms and patient-reported outcomes are considered alongside inflammatory burden, endoscopic disease activity, disease course, treatment requirements, and health-related quality of life.

Acute severe ulcerative colitis

Acute severe ulcerative colitis (ASUC) is identified clinically using the Truelove-Witts framework. The classic definition is ≥6 bloody stools/day plus at least one marker of systemic toxicity, such as tachycardia, fever, anemia, or an elevated inflammatory marker.

ASUC represents an emergency presentation requiring prompt hospital-based assessment and treatment.

Pediatric classification: Paris modification

The Paris classification modifies the Montreal system for pediatric IBD by providing finer age categories, additional upper gastrointestinal location categories, a growth axis, and more explicit characterization of disease behavior.

Feature	Paris modification
Age	A1a: <10 years; A1b: 10 to <17 years
Upper GI location	L4a: proximal to the ligament of Treitz; L4b: distal to the ligament of Treitz
Growth	G0: no growth delay; G1: growth delay
Behavior	Allows explicit pediatric classification of stricturing and penetrating disease

Very-early-onset IBD (VEO-IBD) refers to disease beginning before 6 years of age. Infantile IBD refers to onset before 2 years. These early-onset phenotypes have higher frequencies of IBD-U, extensive colitis, and monogenic or immune-mediated disease than later-onset IBD and may warrant immunologic and genetic evaluation.

Risk stratification and disease phenotype

Classification should not be confused with disease severity. Location, behavior, extent, complications, inflammatory burden, treatment history, and disease impact all contribute to prognosis and treatment selection.

In CD, features associated with a more complicated course include stricturing or penetrating disease, deep or extensive mucosal ulceration, fistula or abscess, previous intestinal resection, ileal or upper gastrointestinal involvement, extensive disease, young age at diagnosis, and smoking. In UC, extensive or deeply ulcerated disease and prior advanced-therapy exposure may identify patients with a more aggressive disease course.

Modern disease-severity frameworks increasingly integrate multiple domains rather than assigning a static mild/moderate/severe label.

Microscopic colitis: related but distinct inflammatory colitis

Microscopic colitis (MC) is characterized clinically by chronic watery, usually non-bloody diarrhea with macroscopically normal or near-normal colonic mucosa. Diagnosis depends on histopathology rather than gross endoscopic appearance.

The two classic subtypes are lymphocytic colitis and collagenous colitis. Incomplete microscopic colitis (MCi) describes cases with intermediate histologic abnormalities that do not meet the full criteria for either classic subtype.

Entity	Typical clinical/endoscopic context	Histologic definition
Lymphocytic colitis	Chronic watery diarrhea; colon often appears normal endoscopically	Increased intraepithelial lymphocytes, classically ≥20 per 100 epithelial cells, without the characteristic thickened collagen band
Collagenous colitis	Chronic watery diarrhea; colon often appears normal endoscopically	Thickened subepithelial collagen band, classically >10 μm, with associated intraepithelial lymphocytosis
Incomplete microscopic colitis (MCi)	Clinical features of MC with incomplete classic histologic criteria	Intermediate collagen-band thickness and/or lymphocytic abnormalities that do not meet criteria for classic MC

MC should not be automatically classified as UC or CD. Because the mucosa may appear normal, histologic sampling is essential when MC is suspected.

Classification in clinical practice

Disease	Core classification system	Main axes
Crohn's disease	Montreal	Age (A), location (L), behavior (B), with perianal modifier (p)
Ulcerative colitis	Montreal	Extent (E) plus multidimensional disease severity
Pediatric CD/UC	Paris modification	Finer age and location categories, growth axis, and pediatric behavior classification
IBD-U	Clinical diagnosis pending further classification	Used when CD vs. UC cannot be confidently assigned
Microscopic colitis	Histologic classification	Lymphocytic, collagenous, or incomplete MC; distinct from CD/UC

Clinically actionable points

• Assign a Montreal classification at diagnosis for CD using age, location, behavior, and the perianal modifier when applicable; classify UC by extent.
• Assess extent and severity separately in UC. Anatomic extent does not substitute for assessment of current inflammatory activity and disease impact.
• Recognize ASUC promptly using the clinical Truelove-Witts framework because it represents a medical emergency.
• Assess for perianal disease in CD because it changes disease classification and has important prognostic and therapeutic implications.
• Use the Paris classification in children rather than applying adult Montreal categories without modification; document growth and developmental status.
• Consider immunologic/genetic evaluation in VEO-IBD and infantile IBD when the clinical phenotype suggests an underlying monogenic or immune disorder.
• Do not classify microscopic colitis as UC or CD. Suspected MC requires appropriate colonic biopsies even when the mucosa appears normal.

Important updates

• Montreal classification superseded the older Vienna framework and introduced the clinically useful perianal modifier while refining age categories.
• Severity assessment has become multidimensional. Modern frameworks integrate symptoms and patient-reported outcomes with inflammatory burden, endoscopic findings, disease course, treatment requirements, and quality-of-life impact rather than relying on a single cross-sectional severity label.
• IBD-U is recognized as a clinically meaningful category. It is particularly relevant in children, where classification may change with longitudinal follow-up.
• Pediatric classification requires additional dimensions. The Paris modification incorporates finer age categories, upper-GI subdivisions, growth, and pediatric disease behavior.
• VEO-IBD has increasing relevance to genetic and immunologic evaluation because monogenic disorders and inborn errors of immunity are enriched in very early disease.
• Interest is shifting toward molecular and location-predominant phenotyping. Genome-scale and endotype-based approaches may eventually complement or modify Montreal classification, but they have not replaced it as the routine clinical standard.

Areas of uncertainty

• Montreal is useful but incomplete. Molecular/endotype-based and location-predominant approaches have been proposed because conventional classification does not fully capture biological heterogeneity or treatment response.
• UC severity instruments serve different purposes and may not agree. Bedside, clinical-trial, endoscopic, and pediatric instruments should not be treated as interchangeable.
• IBD-U may remain unresolved. Some patients do not subsequently reclassify as CD or UC, making long-term classification and certain irreversible decisions more challenging.
• The relationship between microscopic colitis and the broader IBD spectrum remains debated. MC is clinically and histologically distinct from CD/UC, although shared inflammatory and genetic features have prompted discussion of a broader relationship.
• VEO-IBD classification can evolve over time. Changing phenotype and overlap with primary immunodeficiency can complicate initial classification.

High-yield clinical pearls

• Montreal CD = A + L + B, with p for perianal disease and L4 for upper-GI involvement. Location is relatively stable, whereas behavior may evolve.
• Montreal UC = E1 proctitis, E2 left-sided colitis, E3 extensive colitis/pancolitis. The splenic flexure is the principal E2/E3 landmark.
• ASUC = ≥6 bloody stools/day plus systemic toxicity. Recognize it promptly as an emergency.
• IBD-U should not automatically be treated as a permanent diagnosis, particularly in children. Longitudinal reassessment may permit reclassification.
• VEO-IBD (<6 years) and infantile IBD (<2 years) warrant consideration of monogenic and immune-mediated disease.
• Microscopic colitis can have a normal-appearing colon. Histologic biopsies are therefore necessary when it is suspected.

Common pitfalls

• Classifying UC by symptoms alone rather than documenting the anatomic extent of disease.
• Missing the L4 modifier or perianal modifier in CD, resulting in incomplete phenotypic classification.
• Calling all colonic IBD UC. Crohn's colitis and IBD-U can overlap clinically and histologically with UC, particularly early in the disease course.
• Treating IBD-U as a fixed diagnosis without longitudinal reassessment.
• Applying adult Montreal criteria to young children without the pediatric Paris modifications.
• Diagnosing microscopic colitis without histology or excluding it because the colon appears normal endoscopically.

Epidemiology and disease burden

Inflammatory bowel disease (IBD), comprising Crohn disease (CD), ulcerative colitis (UC), and IBD-unclassified, is a chronic inflammatory disorder with a rapidly evolving global epidemiology. IBD prevalence is highest in North America, Europe, and Oceania, while incidence is increasingly rising in newly industrialized regions. In many high-income countries, incidence has stabilized or declined while prevalence continues to increase because of population growth, aging, improved survival, and accumulation of affected individuals.

Global epidemiology

IBD is now a global disease. The highest reported population prevalences have historically been observed in Europe and North America, with reported UC prevalence up to 505 per 100,000 in Norway and CD prevalence up to 322 per 100,000 in Germany. North American estimates include UC prevalence of approximately 286 per 100,000 and CD prevalence of approximately 319 per 100,000.

Global Burden of Disease (GBD) analyses demonstrate substantial worldwide disease burden. The GBD 2017 analysis estimated approximately 6.8 million people living with IBD worldwide, with the highest age-standardized prevalence in high-income North America.

The GBD 2019 analysis estimated approximately 4.9 million prevalent cases globally, with China and the United States among the countries with the largest absolute numbers of cases. Differences between GBD estimates and population-based registry studies may reflect differences in methodology, case ascertainment, and availability of primary data.

United States

Contemporary US claims-based data estimated an age-, sex-, and insurance-standardized IBD prevalence of approximately 721 per 100,000, including approximately 378 per 100,000 for UC and 305 per 100,000 for CD. These estimates corresponded to approximately 2.39 million Americans with IBD, including approximately 1.25 million with UC and 1.01 million with CD. Standardized IBD incidence was approximately 10.9 per 100,000 person-years, with incidence peaking during the third decade of life.

Temporal trends and epidemiologic transition

The epidemiology of IBD can be conceptualized as an evolving transition from emergence of disease to acceleration of incidence, subsequent compounding of prevalence, and ultimately prevalence equilibrium.

In many early-industrialized Western countries, IBD incidence has stabilized or declined, whereas prevalence continues to increase. This distinction reflects the cumulative effect of previously rising incidence, low disease-specific mortality, improved survival, and population aging.

In newly industrialized regions, including parts of Asia, Latin America, Africa, and the Middle East, incidence has increased substantially during the epidemiologic transition. Historical population-based data demonstrate particularly rapid increases in several countries, although the magnitude of increase varies by region and study methodology.

The increasing prevalence of IBD represents an important healthcare-system burden even when age-standardized incidence is stable or declining.

Absolute burden versus age-standardized rates

Absolute numbers of IBD cases may increase while age-standardized incidence, prevalence, mortality, or disability rates remain stable or decline. This apparent divergence is largely attributable to population growth and aging.

GBD analyses have demonstrated declining or stable age-standardized burden measures in several populations despite continued increases in absolute case numbers.

Clinical interpretation: a stable age-standardized incidence rate does not indicate a stable healthcare burden. The absolute number of patients requiring long-term surveillance, treatment, hospitalization, and complication management may continue to increase.

Geographic distribution

Western Europe, North America, and Oceania continue to have among the highest age-standardized IBD rates. In contrast, the absolute number of new cases is increasingly concentrated in highly populated countries in Asia and other rapidly industrializing regions.

This creates an important distinction between rate and absolute burden: Western countries may retain higher standardized rates, whereas populous Asian countries may account for a growing proportion of the world's total number of patients and incident cases.

Age distribution

IBD can occur at any age, but onset most commonly occurs in young adulthood. A bimodal distribution has historically been described, with a major peak during the second to fourth decades of life and a smaller later peak around the fifth to sixth decades.

Pediatric and very-early-onset IBD represent an important epidemiologic subgroup. Incidence in children has continued to increase in some populations, including settings in which adult incidence has plateaued.

Sex distribution

Sex differences in IBD epidemiology are modest and vary according to disease, geographic region, age, and outcome measure.

In many Western populations, CD shows a modest female predominance after adolescence, whereas male predominance has been reported in several Asian populations. GBD analyses have also demonstrated differences between males and females that vary according to whether incidence, prevalence, mortality, or disability is measured.

Race and ethnicity

In contemporary US claims-based data, standardized IBD prevalence was highest among White Americans, followed by Black, Hispanic, and Asian Americans. Reported prevalence estimates were approximately 812, 504, 458, and 403 per 100,000, respectively.

Differences in IBD phenotype and disease burden across racial and ethnic groups are influenced by geography, migration, environmental exposures, socioeconomic factors, access to healthcare, and other determinants. Epidemiologic differences should therefore not be interpreted as evidence that IBD is restricted to any racial or ethnic population.

Socioeconomic and sociodemographic distribution

High and high-middle sociodemographic-index regions currently carry a large proportion of the global IBD burden. At the same time, low- and middle-sociodemographic-index regions have experienced substantial increases in incidence and prevalence as countries undergo socioeconomic and environmental transition.

Disease burden and mortality

IBD is characterized by substantial chronic morbidity despite relatively low disease-specific mortality. GBD 2017 data showed a decline in the global age-standardized mortality rate from approximately 0.61 to 0.51 per 100,000 between 1990 and 2017, while years lived with disability increased substantially.

The resulting epidemiologic pattern is one of a chronic disease with relatively low mortality but a persistent and increasing population-level burden of disability and healthcare utilization.

Environmental epidemiologic associations

Environmental factors are associated with IBD risk and may contribute to geographic and temporal differences in disease occurrence. The strength and consistency of individual associations vary, and many environmental findings remain observational rather than definitively causal.

Epidemiologic factor	Reported association	Clinical interpretation
Smoking	Increases risk of CD; historically associated with lower risk of UC	The divergent association between CD and UC should be recognized. Smoking cessation remains appropriate for overall health and should not be interpreted as a treatment for UC.
Urban living	Associated with increased IBD risk	May reflect differences in diet, pollution, lifestyle, socioeconomic factors, and other environmental exposures.
Antibiotic exposure	Associated with increased IBD risk in observational studies	Association may vary by age, exposure pattern, and disease phenotype.
Physical activity	Associated with lower CD risk in epidemiologic studies	Evidence is primarily observational.
Breastfeeding	Associated with lower IBD risk in some analyses	Evidence supports an epidemiologic association rather than definitive prevention of IBD.
Vitamin D status	Low vitamin D status has been associated with IBD risk	Causal effects and the benefit of supplementation for primary prevention remain uncertain.
Diet and dietary exposures	Several dietary patterns and foods have been associated with IBD risk	Associations are heterogeneous and should not be interpreted as proof of causality.

The broader environmental exposome includes dietary, microbial, medication, pollution, lifestyle, and early-life exposures. Current evidence supports an association between multiple environmental exposures and IBD, but causal attribution for individual exposures remains incompletely established.

Smoking should not be recommended as a therapeutic intervention for UC. The apparent inverse epidemiologic association between smoking and UC is distinct from the established health risks of tobacco exposure and the adverse association with CD.

Projected future burden

Contemporary modeling suggests that IBD burden will continue to increase in absolute terms over coming decades, particularly as populations age and disease becomes more prevalent in countries undergoing socioeconomic transition. Some Western populations may approach a prevalence of approximately 1% by 2030.

GBD-based projections through 2040-2050 generally predict continued increases in absolute numbers of affected individuals despite stable or declining age-standardized rates in several populations. These projections are model-based and should not be interpreted as equivalent to direct population surveillance.

Epidemiologic limitations

Several limitations should be considered when interpreting contemporary IBD epidemiology:

• Population-based studies and GBD estimates are not interchangeable. GBD estimates are modeled and may be particularly uncertain in regions with limited primary epidemiologic data.
• Ascertainment bias may contribute to apparent increases in newly industrialized regions as diagnostic access, awareness, and healthcare utilization improve.
• Absolute case counts and age-standardized rates answer different questions and should not be interpreted interchangeably.
• Environmental associations do not establish causality. Many reported risk factors are derived from observational studies and may be affected by recall bias, reverse causation, residual confounding, or differences in exposure measurement.

High-yield epidemiologic points

• IBD is a global disease and is no longer appropriately characterized as predominantly Western.
• The highest standardized rates remain concentrated in North America, Europe, and Oceania, while the absolute global burden is increasingly shifting toward populous Asian and other newly industrialized regions.
• In many Western countries, incidence has stabilized or declined while prevalence continues to increase.
• Absolute case numbers can rise despite stable or declining age-standardized rates because of population growth and aging.
• IBD most commonly begins in young adulthood, although it can occur at any age.
• Pediatric and very-early-onset IBD represent an important and, in some populations, increasing epidemiologic subgroup.
• Environmental factors contribute to geographic and temporal variation in IBD risk, but most individual associations remain observational.

Pathophysiology and Causes

Inflammatory bowel disease (IBD) arises from dysregulated mucosal immune activation against the intestinal microbiota in a genetically susceptible host exposed to permissive environmental factors. No single cause is sufficient to explain IBD; disease emerges from interactions among host genetics, the environmental exposome, the intestinal microbiome, epithelial barrier function, and innate and adaptive immunity.

Unifying pathophysiologic model

The current model is a self-reinforcing interaction between four major compartments:

Compartment	Principal mechanisms	Pathophysiologic consequence
Host genetics	Variants affecting microbial sensing, autophagy, epithelial integrity, endoplasmic reticulum stress, cytokine signaling, and immune regulation	Increased susceptibility to abnormal host-microbe interactions
Environment / exposome	Diet, smoking, medications, pollutants, early-life exposures, and other environmental factors	Modifies epithelial integrity, immune maturation, and microbiome composition
Microbiome	Reduced diversity and stability, depletion of beneficial taxa, and expansion of pathobionts	Altered microbial metabolites and immune stimulation
Intestinal barrier	Mucus abnormalities, tight-junction dysfunction, epithelial stress, and Paneth-cell dysfunction	Increased microbial and antigenic translocation
Mucosal immunity	Activation of macrophages, dendritic cells, T cells, innate lymphoid cells, and cytokine networks	Persistent inflammation and tissue injury

Genetic susceptibility and environmental exposures alter host-microbe interactions and epithelial homeostasis. Barrier disruption permits microbial products to access the lamina propria, where innate immune cells initiate inflammatory signaling and adaptive immune responses. Cytokine-driven inflammation then further damages the epithelial barrier and alters the microbiome, producing a self-sustaining inflammatory loop.

The major genetic pathways implicated in IBD converge on microbial sensing, epithelial homeostasis, intracellular bacterial clearance, immune regulation, tissue repair, cellular stress responses, and cytokine signaling.

Genetic susceptibility

Genome-wide association studies have identified more than 240 IBD-associated risk loci, with large-scale sequencing additionally identifying rare, higher-impact coding variants. Heritability is greater for Crohn disease (CD) than ulcerative colitis (UC), but currently identified genetic variants explain only a minority of overall disease variance, emphasizing the importance of environmental and microbial factors.

Pathway	Representative genes	Mechanistic role
Microbial sensing	NOD2 (CARD15)	Intracellular sensing of bacterial muramyl dipeptide and regulation of antimicrobial and inflammatory responses
Autophagy / xenophagy	ATG16L1, IRGM	Intracellular bacterial clearance and interaction with NOD2-mediated microbial sensing
Endoplasmic reticulum stress / unfolded protein response	XBP1, ORMDL3	Maintenance of epithelial and immune-cell homeostasis during cellular stress
IL-23 / type-17 immunity	IL23R, IL12B, STAT3, JAK2	Regulation of IL-23-dependent Th17 and related inflammatory pathways
Epithelial barrier / immune regulation	ECM1, IL10, IL10R	Epithelial integrity and suppression of excessive mucosal inflammation

NOD2 is the strongest established CD susceptibility gene and is particularly associated with ileal disease and complicated disease behavior, including stricturing or penetrating disease and increased surgical risk.

Variants in IL10 and IL10R can cause monogenic very-early-onset IBD, illustrating that severe intestinal inflammation may occasionally result from a single high-impact defect in immune regulation.

Genetic architecture also varies by ancestry. NOD2 and IL23R risk variants are substantially less common in East Asian populations than in populations of European ancestry, illustrating why genetic associations derived from one ancestry cannot automatically be generalized to all populations.

Environment and exposome

Environmental exposures modify IBD susceptibility through effects on epithelial integrity, immune development, microbial composition, and host-microbe interactions. Early life represents an important period because immune and microbiome maturation may establish long-term susceptibility.

Exposure or factor	Reported association	Mechanistic context
Cigarette smoking	Increased CD risk; associated with adverse CD course	Alters immune responses, epithelial function, and microbial composition
Urbanization / Westernized environment	Associated with increased IBD risk	Broad effects on diet, pollutants, lifestyle, and microbial exposure
Antibiotic exposure	Associated with IBD risk	Perturbs intestinal microbial communities
Dietary pattern	Western diet, ultra-processed foods, red meat, and dietary emulsifiers have been implicated	Alters microbial metabolism, epithelial function, and mucosal immune signaling
Air and water pollutants / PFAS	Increasingly implicated in IBD susceptibility	May alter barrier function, immune signaling, and the microbiome
Breastfeeding	Associated with lower IBD risk in epidemiologic studies	May influence early immune and microbiome maturation
Physical activity	Associated with lower IBD risk	Potential effects on immune and metabolic homeostasis
Vitamin D status	Vitamin D deficiency associated with IBD risk	Vitamin D participates in epithelial and immune regulation

The life-course or "multiple hits" model proposes that early environmental exposures establish a susceptibility phenotype upon which later exposures act as additional triggers.

Environmental associations should not be interpreted as single sufficient causes of IBD. Many exposures are correlated with one another and may act through several biological pathways simultaneously.

Microbiome and dysbiosis

IBD is associated with reduced microbial diversity and ecological stability, depletion of several butyrate-producing organisms, and expansion of Proteobacteria and other pathobionts, including adherent-invasive Escherichia coli in subsets of patients.

Rather than a single causative organism, dysbiosis represents an altered ecological state in which the relative abundance and function of commensals and pathobionts change. Reduced production of beneficial microbial metabolites, including short-chain fatty acids, may impair epithelial barrier integrity and regulatory immune function.

A central unresolved issue is whether dysbiosis initiates intestinal inflammation or develops secondarily to inflammation, diet, medications, and altered intestinal physiology. Current evidence supports a bidirectional relationship in which dysbiosis can promote inflammation while inflammation further reshapes the microbial ecosystem.

Intestinal epithelial barrier dysfunction

The intestinal epithelium forms a dynamic barrier between luminal microorganisms and the mucosal immune system. Barrier dysfunction involves abnormalities in the mucus layer, epithelial cells, tight-junction complexes, Paneth cells, and epithelial stress responses.

Increased intestinal permeability can facilitate translocation of microbial products into the lamina propria, activating innate and adaptive immune pathways. Evidence that barrier abnormalities may precede clinically apparent CD and correlate with subsequent disease activity supports a role for barrier dysfunction as an early pathogenic event rather than simply a consequence of established inflammation.

The genetic pathways affecting epithelial homeostasis include tight-junction regulation, Paneth-cell function, autophagy, endoplasmic reticulum stress responses, and cellular repair pathways.

Innate and adaptive immune dysregulation

Microbial products crossing a dysfunctional epithelial barrier activate macrophages, dendritic cells, and other innate immune cells. These cells produce inflammatory mediators including TNF-α, IL-1, IL-6, IL-12, and IL-23, which promote activation and differentiation of pathogenic effector lymphocytes.

The adaptive response includes prominent Th1 and Th17 pathways. Th1 cells produce interferon-γ, while Th17 cells and group 3 innate lymphoid cells contribute IL-17A, IL-17F, and IL-22. Regulatory T-cell mechanisms are insufficient to restore immune homeostasis, allowing chronic inflammation to persist.

Immune pathway	Major mediators	Relevance to IBD
Innate immune activation	Macrophages, dendritic cells, TNF-α, IL-1, IL-6	Initiates and amplifies mucosal inflammation
IL-12 / Th1 pathway	IL-12, IFN-γ	Promotes cell-mediated inflammatory responses
IL-23 / Th17 pathway	IL-23, IL-17A/F, IL-22	Major pathogenic pathway in chronic intestinal inflammation
Regulatory immunity	Regulatory T cells and anti-inflammatory pathways	Normally limits mucosal inflammation; inadequate regulation contributes to chronicity
Cytokine networks	TNF-α, IL-23 and downstream signaling pathways	Sustain communication between immune, epithelial, stromal, and other cells

Cytokine hubs and therapeutic targets

IL-23 has emerged as a central cytokine hub in IBD pathogenesis, together with TNF-α. IL-23 promotes persistence and pathogenicity of type-17 immune responses and coordinates inflammatory signaling across multiple intestinal cell populations.

This pathophysiologic model explains several therapeutic target classes:

Target / pathway	Representative therapy	Mechanistic principle
TNF	Anti-TNF agents	Neutralization of a major proinflammatory cytokine
IL-12/IL-23 p40	Ustekinumab	Blocks the shared p40 subunit of IL-12 and IL-23
IL-23 p19	Risankizumab, mirikizumab, guselkumab	Selective inhibition of IL-23 signaling
JAK-STAT signaling	Tofacitinib and other JAK inhibitors	Blocks intracellular signaling downstream of multiple cytokine receptors
α4β7 integrin	Vedolizumab	Reduces gut-selective lymphocyte trafficking

The therapeutic success of these pathways demonstrates that IBD is not driven by a single linear cytokine cascade but by interconnected immune networks.

An important example of this complexity is IL-17A. Although IL-17A participates in intestinal inflammation, it also contributes to epithelial barrier maintenance and repair; direct IL-17A blockade has therefore been associated with worsening of CD rather than therapeutic benefit.

Pharmacogenetic implications

Some genetic findings have direct clinical relevance to treatment selection or toxicity prevention.

Genetic factor	Clinical relevance
TPMT	Reduced activity increases susceptibility to thiopurine-induced myelosuppression
NUDT15	Variants increase susceptibility to thiopurine-induced leukopenia and other hematologic toxicity
HLA-DQA1*05	Associated with increased immunogenicity to anti-TNF therapy
NOD2	Associated with CD phenotype and disease behavior but is not a routine diagnostic test
IL23R and other IBD susceptibility variants	Important for understanding pathogenesis but generally not established as routine diagnostic or prognostic tests

TPMT and NUDT15 assessment should be incorporated into thiopurine treatment decisions according to the applicable clinical testing strategy. HLA-DQA1*05 may provide information about anti-TNF immunogenicity, but its use should be interpreted in the context of the overall treatment strategy.

Routine NOD2 or broad IBD susceptibility genotyping is not a diagnostic test for IBD and should not be used as a substitute for clinical, endoscopic, histologic, and laboratory assessment.

Monogenic and very-early-onset IBD

Very-early-onset IBD can occasionally result from monogenic defects in immune regulation, epithelial function, or host defense. Examples include IL-10 and IL-10 receptor defects.

A suspected monogenic disorder is particularly relevant when disease begins very early in life, is unusually severe or refractory, or is accompanied by features suggesting an underlying immunodeficiency or systemic genetic disorder. Such patients may require specialized genetic and immunologic evaluation. In selected monogenic disorders, hematopoietic stem-cell transplantation may be considered in specialized centers.

Clinically actionable implications

• Interpret IBD as a multifactorial disease: avoid attributing disease to a single food, microorganism, genetic variant, or environmental exposure.
• Use pathophysiology to understand treatment targets: TNF, IL-23, JAK-STAT signaling, and α4β7-mediated trafficking represent clinically actionable components of the inflammatory network.
• Consider pharmacogenetics when relevant to therapy: TPMT and NUDT15 are clinically relevant before or around thiopurine use, while HLA-DQA1*05 may inform anti-TNF immunogenicity considerations.
• Do not use susceptibility genes as diagnostic tests: NOD2 and other IBD risk variants indicate biological susceptibility rather than establishing a diagnosis.
• Address modifiable environmental exposures: smoking cessation is particularly relevant to CD, and overall lifestyle and nutritional counseling should be individualized. Environmental associations should not be presented as single proven causes of IBD.
• Recognize possible monogenic IBD in appropriate patients: very-early-onset or unusually severe disease may warrant genetic and immunologic evaluation.

Important updates from older teaching

• The number of recognized IBD-associated genetic loci has increased from the older estimate of several dozen to more than 240, with rare high-impact variants identified through sequencing.
• IL-23 is now recognized as a major pathogenic cytokine hub, changing the mechanistic interpretation of therapies targeting the IL-12/23 pathway and supporting selective IL-23p19 inhibition.
• Genetic susceptibility explains only a minority of overall disease variance, emphasizing interactions among genetics, environment, microbiome, and host immunity.
• Epithelial barrier dysfunction is increasingly viewed as an early component of IBD pathogenesis rather than simply a downstream consequence of inflammation.
• Pharmacogenetic information, particularly TPMT/NUDT15 and HLA-DQA1*05, has become clinically relevant to selected therapeutic decisions.
• IBD genetic architecture differs among ancestral populations, limiting direct extrapolation of findings from predominantly European cohorts.

Areas of uncertainty

• Dysbiosis: whether microbial alterations primarily initiate IBD, result from inflammation, or both remains unresolved.
• Barrier dysfunction: evidence supports an early role for increased permeability, but the extent to which barrier abnormalities initiate disease versus amplify established inflammation remains incompletely defined.
• Causal genetic variants: most IBD-associated loci do not yet have a fully established causal variant and molecular mechanism.
• Molecular endotypes: it remains uncertain whether clinically defined UC and CD represent a limited number of distinct molecular endotypes or a continuous spectrum of overlapping biological states.
• Microbiome-directed therapy: the biological importance of dysbiosis does not establish that microbiome manipulation is an effective routine treatment; interventions such as FMT, probiotics, and metabolite-based approaches remain areas of investigation.

High-yield clinical pearls

• NOD2 is a major CD susceptibility gene, not a diagnostic test. Its strongest clinical associations are with ileal and complicated CD phenotypes.
• TPMT and NUDT15 matter for thiopurine toxicity. Reduced activity can markedly increase the risk of myelosuppression.
• IL-23 is a central inflammatory hub in both UC and CD and is an important therapeutic target.
• Cytokines can have context-dependent effects. IL-17A illustrates that a cytokine involved in inflammation may simultaneously support epithelial barrier integrity, explaining why direct blockade can worsen CD.
• IBD is not caused by a single "bad bacterium." Dysbiosis is part of a complex host-microbe interaction and may be both a driver and consequence of inflammation.
• Genetic susceptibility is only one component of IBD risk. Environmental exposures and microbiome-host interactions substantially modify disease development and phenotype.

Common pitfalls

• Attributing IBD to one environmental exposure or food. Most exposures modify susceptibility or disease biology rather than acting as sufficient causes.
• Calling infectious or other forms of colitis causes of IBD. Tuberculosis, amebiasis, C. difficile, radiation injury, vasculitis, and other disorders are important mimics or alternative causes of intestinal inflammation and should not be conflated with IBD pathogenesis.
• Ordering broad genetic panels to diagnose IBD. No common susceptibility variant is sufficiently sensitive or specific to establish the diagnosis.
• Ignoring TPMT/NUDT15 before thiopurine therapy. These variants can materially alter hematologic toxicity risk.
• Assuming dysbiosis proves a single microbial cause. Current evidence supports a complex bidirectional relationship between inflammation and the microbiome.
• Extrapolating genetic associations across ancestries without qualification. IBD genetic architecture differs among populations.
• Using mechanistic plausibility as proof of therapeutic efficacy. A biologically plausible microbiome, cytokine, or barrier target requires clinical evidence before being considered established therapy.

Clinical presentation

IBD has a heterogeneous, relapsing-remitting clinical presentation that varies according to disease location, extent, inflammatory activity, and the presence of stricturing, penetrating, or extraintestinal disease. Ulcerative colitis (UC) typically presents with rectal bleeding, increased stool frequency, urgency, and tenesmus, whereas Crohn disease (CD) more often presents with abdominal pain, diarrhea, fatigue, weight loss, and disease-specific complications such as strictures, fistulae, abscesses, or perianal disease.

Ulcerative colitis

UC usually presents with rectal bleeding, which occurs in more than 90% of patients. Other common features include increased stool frequency, loose stools, rectal urgency, tenesmus, mucus discharge, nocturnal defecation, fecal incontinence, and crampy abdominal pain, often localized to the left lower quadrant and relieved by defecation.

Clinical presentation varies with disease extent:

Disease extent	Typical clinical presentation
Proctitis	Rectal bleeding, urgency, tenesmus, mucus, and rectal discomfort. Constipation or difficult stool passage may occur because of distal inflammation and proximal stool retention.
Left-sided colitis	Bloody diarrhea, increased stool frequency, urgency, tenesmus, and crampy abdominal pain.
Extensive colitis / pancolitis	More frequent bloody diarrhea, abdominal pain, systemic symptoms, weight loss, and features of more severe intestinal inflammation.

Abdominal pain is generally less prominent in UC than in CD.

A minority of patients present with an acute severe UC (ASUC) phenotype, characterized by frequent bloody stools together with systemic features of toxicity. The Truelove-Witts criteria identify severe disease using stool frequency and systemic parameters including tachycardia, fever, anemia, and elevated inflammatory markers.

Crohn disease

CD can involve any part of the gastrointestinal tract from the mouth to the anus. The typical presentation is chronic diarrhea, abdominal pain, weight loss, and fatigue, particularly in younger adults.

Clinical manifestations vary substantially according to disease location and phenotype:

CD phenotype	Typical clinical presentation
Inflammatory / luminal	Chronic diarrhea, abdominal pain, fatigue, anorexia, weight loss, and other systemic features of intestinal inflammation. Diarrhea is often non-bloody.
Stricturing	Postprandial abdominal pain, bloating, nausea, vomiting, early satiety, abdominal distension, and recurrent obstructive symptoms.
Penetrating / fistulizing	Enteroenteric, enterovesical, enterocutaneous, or rectovaginal fistulae; manifestations depend on the involved organs and may include recurrent urinary infections, pneumaturia, fecaluria, passage of stool through the vagina, or an inflammatory mass/abscess.

Clinical presentation by anatomic location

The site of intestinal involvement can produce characteristic symptoms.

Disease location	Common clinical manifestations
Ileal / ileocolonic CD	Right lower quadrant abdominal pain, diarrhea, weight loss, fatigue, and nutritional deficiencies. Stricturing disease may produce obstructive symptoms.
Colonic CD	Diarrhea, abdominal pain, rectal bleeding, and urgency; perianal disease may coexist.
Upper gastrointestinal CD	Epigastric pain, nausea, vomiting, dyspepsia, or early satiety.
Perianal CD	Perianal pain, drainage, fissures, ulcers, skin tags, stenosis, fistulae, and abscesses.

Perianal Crohn disease

Perianal disease occurs in approximately one-fifth of patients with CD and includes skin tags, fissures, ulcers, anal stenosis, fistulae, and abscesses. Perianal manifestations may precede or accompany the intestinal diagnosis, and fistulizing perianal disease can occasionally be the sole initial manifestation.

Recurrent or complex perianal fistulae should raise strong suspicion for CD. The presence of strictures, fistulae, or abscesses at or near diagnosis may indicate substantial bowel damage despite relatively limited gastrointestinal symptoms.

Common presenting symptoms in Crohn disease and ulcerative colitis

Symptoms overlap substantially between UC and CD, but their relative frequency and pattern can help establish the clinical phenotype.

Feature	Ulcerative colitis	Crohn disease
Rectal bleeding	Very common; characteristic of colonic disease	Less common overall; may occur with colonic involvement
Diarrhea	Usually frequent and often bloody	Common; often non-bloody
Urgency / tenesmus	Very common, particularly with distal disease	May occur with colonic or rectal involvement
Abdominal pain	Usually crampy and less dominant	Very common and may be prominent
Fatigue	Common	Very common
Weight loss	More prominent with extensive or severe disease	Common, particularly with active small-bowel disease
Nocturnal stools	May occur with active disease	May occur with active disease
Obstructive symptoms	Uncommon at presentation	Suggest stricturing CD
Perianal disease	Unusual	Characteristic of CD

Community-based inception-cohort data found that fatigue and abdominal pain were among the predominant symptoms in CD, whereas passage of blood and loose or watery stools predominated in UC.

Extraintestinal manifestations

Extraintestinal manifestations (EIMs) occur in a substantial proportion of patients with IBD and may precede gastrointestinal symptoms. Their presence can therefore be an important clue to underlying IBD.

Organ system	Clinical manifestations	Relationship to intestinal activity
Musculoskeletal	Peripheral arthritis, enthesitis, axial arthropathy, sacroiliitis, and ankylosing spondylitis	Peripheral arthritis may parallel intestinal activity; axial disease may follow an independent course
Skin / oral	Erythema nodosum, pyoderma gangrenosum, aphthous stomatitis, and Sweet syndrome	Erythema nodosum often parallels intestinal activity; other manifestations may be independent
Eye	Episcleritis, scleritis, and anterior uveitis	Episcleritis often parallels intestinal activity; uveitis may occur independently
Hepatobiliary	Primary sclerosing cholangitis and other hepatobiliary manifestations	May follow an independent course from intestinal activity

EIMs may be the first clinical manifestation of IBD. Patients presenting with otherwise unexplained arthritis, erythema nodosum, uveitis, oral ulceration, or other characteristic EIMs should be assessed for accompanying gastrointestinal symptoms and other features suggestive of IBD.

Pediatric presentation

Children may present with the same intestinal symptoms as adults, including diarrhea, rectal bleeding, abdominal pain, and weight loss. However, growth failure and delayed puberty are particularly important presenting features of pediatric CD and may precede prominent gastrointestinal symptoms.

Important pediatric presenting features include:

Feature	Clinical significance
Poor weight gain / weight loss	May reflect reduced intake, malabsorption, or inflammatory disease activity.
Linear growth failure	May be a presenting feature of CD and can occur despite relatively subtle gastrointestinal symptoms.
Delayed puberty	May accompany chronic inflammatory disease and nutritional impairment.
Abdominal pain / diarrhea	Common intestinal manifestations, but may be absent or understated in children with growth impairment.
Rectal bleeding	Particularly suggestive of colonic IBD and common in pediatric UC.

Up to two-thirds of children with CD may have impaired weight and approximately one-third may have impaired height at diagnosis. Growth impairment is multifactorial and may reflect undernutrition, reduced intake, inflammatory cytokines, and treatment effects. Weight recovery generally occurs faster than recovery of linear growth, emphasizing the clinical importance of recognizing growth impairment early.

Red flags suggesting inflammatory bowel disease

The following clinical features should increase suspicion for IBD rather than a purely functional gastrointestinal disorder:

Red flag	Clinical implication
Rectal bleeding	Particularly recurrent or unexplained bleeding, especially with diarrhea or urgency
Nocturnal diarrhea or stools	Suggests an organic inflammatory process rather than a purely functional disorder
Unintentional weight loss	Suggests active systemic or intestinal disease
Growth failure or delayed puberty	Important pediatric clue, particularly for CD
Persistent abdominal pain with diarrhea	Raises concern when accompanied by other inflammatory features
Perianal fistula, abscess, fissure, or inflammatory skin tags	Strongly suggests CD
Extraintestinal manifestations	Arthritis, erythema nodosum, uveitis, or other characteristic EIMs may precede intestinal symptoms
Persistent fatigue or systemic symptoms	May accompany active IBD and should not be dismissed when combined with gastrointestinal symptoms

Symptoms alone do not reliably reflect the degree of intestinal inflammation; patients may have substantial structural bowel damage despite a relatively modest symptom burden.

Clinical presentation: high-yield distinctions

• UC: think bloody diarrhea, urgency, tenesmus, mucus, and rectal bleeding.
• CD: think chronic diarrhea, abdominal pain, fatigue, weight loss, and a broader range of obstructive, fistulizing, or perianal manifestations.
• Perianal fistulae or abscesses strongly favor CD.
• Proctitis may present with urgency, tenesmus, and constipation rather than typical diarrhea.
• Growth failure or delayed puberty may be the dominant manifestation of pediatric CD.
• EIMs may precede gastrointestinal symptoms and can provide the first clue to IBD.
• Fatigue is a common and clinically important symptom in both UC and CD.
• A symptom pattern suggestive of IBD should not be attributed to a functional disorder solely because symptoms are intermittent or inflammatory laboratory abnormalities are absent.

Common clinical pitfalls

• Attributing chronic diarrhea or abdominal pain to a functional disorder without considering bleeding, nocturnal symptoms, weight loss, growth failure, perianal disease, or EIMs.
• Failing to distinguish the predominantly bloody, distal presentation of UC from the more heterogeneous presentation of CD.
• Overlooking perianal disease in a patient with suspected CD.
• Missing pediatric CD because growth failure or delayed puberty is attributed to constitutional variation.
• Treating fatigue as nonspecific when it occurs together with persistent gastrointestinal symptoms or other systemic features.
• Assuming that mild gastrointestinal symptoms exclude clinically important CD, given the potential disconnect between symptoms and structural bowel damage.

Diagnosis

Diagnosis requires integration of:

• Clinical history.
• Physical examination.
• Laboratory inflammatory markers.
• Stool biomarkers.
• Endoscopy with histology.
• Cross-sectional imaging when indicated.

No single test independently establishes the diagnosis.

Fecal calprotectin should be interpreted according to clinical context and pretest probability.

Important principles:

• Biopsies obtained from ulcer bases provide the highest diagnostic yield.
• Hematoxylin-eosin staining has limited sensitivity.
• Immunohistochemistry, rapid viral culture, or tissue PCR are preferred diagnostic methods.

Diagnostic workup

Investigation	Main purpose
CBC	Evaluate anemia, leukocytosis, and thrombocytosis
CMP	Assess renal function, electrolytes, liver tests, and nutritional status
CRP	Assess systemic inflammatory activity and monitor trends
ESR	Additional marker of systemic inflammation
Fecal calprotectin	Assess intestinal inflammation and help distinguish active IBD from functional symptoms
Stool pathogen testing	Exclude infectious causes of diarrhea
Clostridioides difficile testing	Important in patients with active colitis or worsening symptoms
Colonoscopy with biopsies	Establish diagnosis, determine disease extent, and assess mucosal activity
Upper endoscopy	Consider when upper gastrointestinal involvement is suspected
MR enterography or CT enterography	Evaluate small-bowel Crohn disease and transmural complications
Pelvic MRI	Evaluate suspected perianal fistulizing Crohn disease

Biomarker monitoring: Fecal calprotectin

Fecal calprotectin is a neutrophil-derived stool biomarker used to assess intestinal inflammation.

Clinical applications include:

• Differentiating IBD from functional gastrointestinal disorders.
• Monitoring inflammatory activity.
• Supporting treatment decisions.

The AGA framework is based on clinical probability.

Clinical scenario	Threshold	Interpretation	Action
Symptomatic remission with low probability of inflammation	Approximately ≤150 µg/g	Suggests absence of active inflammation	Avoid routine endoscopy
Moderate-to-severe symptoms with high probability	>150–250 µg/g	Supports active inflammation	Treatment decisions may proceed without routine endoscopy
Mild symptoms with intermediate probability	Any cutoff	Accuracy insufficient	Perform endoscopy

In established ulcerative colitis for detection of Mayo endoscopic subscore 2–3:

• 50 ± 50 µg/g:
  • Sensitivity approximately 78%.
  • Specificity approximately 57%.

• 150 ± 50 µg/g:
  • Sensitivity approximately 71%.
  • Specificity approximately 69%.

• 250 ± 50 µg/g:
  • Sensitivity approximately 67%.
  • Specificity approximately 73%.

ACG guideline interpretation:

• Pooled sensitivity for endoscopic inflammation: approximately 87%.
• Pooled specificity: approximately 77%.
• Fecal calprotectin around 321 µg/g during clinical remission predicts relapse risk at 6 and 12 months.

A normal fecal calprotectin does not exclude active disease when clinical suspicion remains high

^Treatment

Treatment options in IBD can be broadly divided into conventional therapies, biologic therapies, targeted small molecules, and surgery when indicated. The choice of treatment depends on disease severity, phenotype, location, prognostic risk, previous treatment exposure and response, safety considerations, and patient preferences.

Treatment class	Examples	Main role in IBD	Key considerations
5-ASA	Mesalamine, sulfasalazine, balsalazide	Mild-to-moderate ulcerative colitis; induction and maintenance	Limited role in Crohn disease
Corticosteroids	Prednisone, methylprednisolone, budesonide	Rapid induction of remission	Not for maintenance; minimize cumulative exposure
Thiopurines	Azathioprine, 6-mercaptopurine	Steroid-sparing and selected maintenance therapy	Slow onset; monitor for cytopenias, hepatotoxicity, and malignancy
Methotrexate	Methotrexate	Selected Crohn disease maintenance/steroid-sparing therapy	Teratogenic; not a rapid induction agent
Anti-TNF agents	Infliximab, adalimumab, certolizumab, golimumab	Moderate-to-severe UC and CD; fistulizing CD; ASUC rescue	Infection screening; immunogenicity; therapeutic drug monitoring
Anti-integrin	Vedolizumab	Moderate-to-severe UC and CD	Gut-selective mechanism
IL-12/23 inhibitor	Ustekinumab	Moderate-to-severe UC and CD	Targets p40 subunit shared by IL-12 and IL-23
Selective IL-23 inhibitors	Risankizumab, guselkumab, mirikizumab	Moderate-to-severe UC and CD	Target IL-23 p19 subunit
JAK inhibitors	Tofacitinib, upadacitinib	Moderate-to-severe UC and selected CD	Oral; rapid onset; consider infection, VTE, MACE, and malignancy risks
S1P receptor modulators	Ozanimod, etrasimod	Moderate-to-severe UC	Oral; monitor cardiac, ophthalmic, hepatic, and infectious risks

Treat-to-target strategy

Symptoms correlate imperfectly with inflammatory burden and should not be used as the sole measure of treatment success. A substantial proportion of patients who report clinical remission may have persistent objective inflammation.

The treat-to-target strategy therefore uses predefined clinical, biochemical, endoscopic, and patient-centered targets to guide longitudinal treatment.

The overarching therapeutic goal is sustained steroid-free remission with control of objective intestinal inflammation and prevention of progressive bowel damage, disability, hospitalization, surgery, and colorectal neoplasia.

Treatment should be individualized according to:

• Disease severity.
• Disease location.
• Disease behavior.
• Risk of progression.
• Prior treatment exposure and response.
• Biomarker and endoscopic activity.
• Comorbidities and safety considerations.
• Patient preferences and treatment burden.

STRIDE-II treatment targets

The STRIDE-II framework organizes treatment targets according to their expected time to achievement.

Target domain	Time horizon	Examples
Clinical symptoms	Short term	Resolution or substantial improvement in diarrhea, rectal bleeding, abdominal pain, urgency, and other disease-related symptoms
Steroid exposure	Short term	Discontinuation of corticosteroids and avoidance of steroid dependence
CRP	Short to intermediate term	Normalization when previously elevated
Fecal calprotectin	Intermediate term	Reduction toward approximately 100–250 µg/g
Endoscopic response	Intermediate to long term	Reduction in ulceration and inflammatory lesions
Endoscopic healing	Long term	Absence or near-absence of visible inflammatory lesions
Quality of life	Long term	Restoration of normal functioning and patient-reported quality of life
Disability	Long term	Prevention or reversal of IBD-related disability
Growth in children	Long term	Restoration of normal growth and pubertal development
Transmural healing	Long term / prognostic	Resolution of bowel-wall inflammation on cross-sectional imaging or intestinal ultrasound

Immediate and short-term targets

Initial treatment should produce:

• Improvement in disease-related symptoms.
• Reduction in stool frequency.
• Resolution of rectal bleeding when present.
• Improvement in abdominal pain and urgency.
• Avoidance of hospitalization and corticosteroid exposure when possible.

Symptomatic improvement is an important target but is not sufficient to establish inflammatory remission.

Intermediate targets

Intermediate targets include:

• Normalization of CRP when elevated.
• Substantial reduction in fecal calprotectin.
• Objective reduction in endoscopic inflammation.
• Avoidance of corticosteroid dependence.
• Improvement in nutritional status and anemia when related to active disease.

Long-term targets

Long-term targets include:

• Sustained steroid-free clinical remission.
• Endoscopic healing.
• Prevention of strictures, fistulas, abscesses, and other bowel damage.
• Prevention of hospitalization and surgery.
• Restoration of quality of life.
• Prevention of disability.
• Normal growth and development in pediatric patients.

Histologic healing in ulcerative colitis and transmural healing in Crohn disease may provide additional prognostic information, although they are not universally established as mandatory treatment targets.

Endoscopic definitions

Crohn disease

Endoscopic response is commonly defined as:

• >50% decrease in the SES-CD or CDEIS.

Endoscopic remission may be defined using validated scores such as:

• SES-CD ≤2.
• CDEIS ≤3.

Ulcerative colitis

Endoscopic healing refers to substantial resolution of visible inflammatory mucosal lesions.

Common trial definitions include:

• Mayo Endoscopic Score (MES) 0 or 1.
• Ulcerative Colitis Endoscopic Index of Severity (UCEIS) ≤1.

STRIDE-II places greater emphasis on complete endoscopic healing when feasible, particularly MES 0 in ulcerative colitis.

Clinical symptoms versus objective inflammation

Patients may have:

Clinical state	Objective inflammatory activity	Interpretation
Symptoms active	Active	Active IBD; assess treatment response
Symptoms improved	Active	Symptom–inflammation discordance; do not assume remission
Symptoms active	Absent	Consider IBS-like symptoms, bile acid diarrhea, infection, stricture, pelvic floor dysfunction, or other noninflammatory causes
Symptoms absent	Absent	Objective remission
Symptoms absent	Active	Silent or subclinical inflammation; treatment optimization may be required

Treat-to-target trial evidence

CALM trial — biomarker-driven tight control

CALM was an open-label phase 3 randomized trial evaluating biomarker-driven treatment escalation compared with symptom-driven management in patients with active Crohn disease.

The study included patients with:

• CDEIS >6.
• CDAI 150–450.
• No previous immunomodulator or biologic exposure.

The tight-control strategy used predefined escalation triggers including:

• Fecal calprotectin ≥250 µg/g.
• CRP ≥5 mg/L.
• CDAI ≥150.
• Recent corticosteroid requirement.

The primary endpoint was mucosal healing defined by:

• CDEIS <4.
• Absence of deep ulcerations.

Outcome	Tight control	Clinical management
Mucosal healing	46%	30%
Deep remission	Higher by 14.5 percentage points	Reference
Biologic remission	Higher by 14.5 percentage points	Reference
Treatment-emergent adverse events	86%	82%

Primary mucosal-healing outcome:

• Risk difference 16.1%.
• 95% CI 3.9–28.3.
• p=0.010.

Clinical implication:

Biomarker-driven monitoring demonstrated improved objective mucosal outcomes compared with symptom-driven management in this early-treatment Crohn disease population.

Limitations included:

• Open-label design.
• Site-read endoscopy.
• Selected treatment-naive population.
• Relatively short follow-up.

STARDUST trial — endoscopy-driven ustekinumab strategy

STARDUST evaluated an endoscopy-driven treat-to-target strategy compared with standard clinical management in patients with Crohn disease receiving ustekinumab.

The trial did not demonstrate a statistically significant difference in the primary endoscopic response endpoint.

Reported outcomes included:

• Endoscopic response: approximately 38% versus 30%.
• Endoscopic remission: approximately 11% versus 15%.
• Clinical remission: approximately 62% versus 70%.

The primary endpoint was not statistically significant.

Important limitation: The trial included a maintenance strategy using every-12-week ustekinumab dosing in the study protocol. This should not be interpreted as evidence that every-12-week dosing is the standard U.S. FDA-approved maintenance regimen.

REACT-2 trial — endoscopic versus symptom-based management

REACT-2 compared a strategy incorporating endoscopic ulcer assessment with conventional symptom-based management in Crohn disease.

The study did not demonstrate a significant improvement in its primary endpoint with the endoscopy-based strategy.

Post hoc analyses suggested possible benefit in selected patients with:

• Elevated CRP.
• Baseline ulceration.

These subgroup findings should be considered hypothesis-generating.

REACT-2 was not an early-combination-therapy trial.

Pooled treat-to-target evidence

Pooled analyses of treat-to-target trials have not established that mandatory endoscopic escalation provides a universal clinical-remission advantage over strategies using symptoms and biomarkers.

One pooled analysis reported clinical remission of:

• 63.2% versus 57.3%.
• RR 1.04.
• 95% CI 0.78–1.39.

The evidence supports objective monitoring, while the optimal frequency and intensity of repeated endoscopic assessment remain areas of ongoing investigation.

5-aminosalicylates

5-aminosalicylates (5-ASA), particularly mesalamine, are primarily used for ulcerative colitis.

Ulcerative colitis

5-ASA therapy is appropriate for many patients with mild-to-moderate UC.

Treatment selection depends on disease extent.

UC extent	Common approach
Proctitis	Rectal mesalamine suppository
Left-sided colitis	Rectal mesalamine plus oral mesalamine when needed
Extensive colitis	Oral mesalamine; rectal therapy may be added when distal symptoms are prominent

Clinical considerations:

• Mesalamine is effective for induction of remission in mild-to-moderate UC.
• Mesalamine is effective for maintenance of remission in UC.
• Rectal therapy provides high local drug exposure in distal disease.
• Combination oral and rectal mesalamine may improve efficacy in left-sided or extensive disease with distal symptoms.

Crohn disease

Oral mesalamine is not recommended for induction or maintenance of moderate-to-severe Crohn disease.

Mesalamine is also not recommended as routine postoperative prophylaxis for Crohn disease.

Corticosteroids

Corticosteroids are primarily induction therapies.

They should not be used as maintenance therapy because cumulative exposure is associated with:

• Osteoporosis.
• Infection.
• Hyperglycemia.
• Hypertension.
• Weight gain.
• Adrenal suppression.
• Cataracts.
• Myopathy.
• Other metabolic complications.

The treatment objective should be transition to an effective steroid-sparing maintenance therapy.

Systemic corticosteroids

Systemic corticosteroids may be used for moderate-to-severe inflammatory activity requiring rapid induction.

Prednisone and intravenous corticosteroids are commonly used depending on disease severity and clinical setting.

Budesonide

Budesonide has extensive first-pass hepatic metabolism and produces less systemic corticosteroid exposure than conventional systemic corticosteroids.

Controlled ileal-release budesonide:

• Typical dose: 9 mg daily.
• Used for selected mild-to-moderate ileocecal Crohn disease.
• Primarily an induction therapy.
• Not appropriate as long-term maintenance therapy.

MMX budesonide may be used as an induction option for selected patients with mild-to-moderate ulcerative colitis.

Steroids should generally be tapered and discontinued as soon as clinically feasible.

Thiopurines and methotrexate

Thiopurines

Azathioprine and 6-mercaptopurine have a role as:

• Steroid-sparing therapy.
• Maintenance therapy in selected patients.
• Combination therapy with anti-TNF agents to reduce immunogenicity in selected patients.

They have a slow onset of action and are not preferred when rapid induction of remission is required.

Important risks include:

• Leukopenia.
• Hepatotoxicity.
• Pancreatitis.
• Infections.
• Nonmelanoma skin cancer.
• Lymphoproliferative disorders.

Pretreatment testing should include TPMT and/or NUDT15 assessment when appropriate.

Methotrexate

Methotrexate may be used in selected Crohn disease patients for steroid-sparing or maintenance therapy.

It is not an induction therapy for severe disease requiring rapid control.

Important adverse effects include:

• Hepatotoxicity.
• Cytopenias.
• Gastrointestinal intolerance.
• Pulmonary toxicity.
• Teratogenicity.

Methotrexate is contraindicated during pregnancy and should be discontinued well before conception.

Biologic therapy

Biologic therapy is used for moderate-to-severe IBD and for selected patients with high-risk disease who may benefit from early advanced therapy.

Major biologic classes include:

• Anti-TNF agents.
• Anti-integrin therapy.
• IL-12/23 inhibition.
• Selective IL-23 inhibition.

Anti-TNF agents

Major agents include:

• Infliximab.
• Adalimumab.
• Certolizumab pegol.
• Golimumab.

Anti-TNF therapy has an established role in both Crohn disease and ulcerative colitis.

Infliximab has particularly strong evidence in:

• Fistulizing Crohn disease.
• Acute severe UC rescue.
• Moderate-to-severe UC.
• Moderate-to-severe Crohn disease.

Important considerations include:

• Tuberculosis screening.
• Hepatitis B screening.
• Infection risk.
• Infusion or injection reactions.
• Immunogenicity.
• Secondary loss of response.

Combination with a thiopurine may improve efficacy and reduce immunogenicity in selected patients.

Anti-integrin therapy

Vedolizumab selectively targets the α4β7 integrin and limits lymphocyte trafficking to the intestinal tract.

It is approved for:

• Moderate-to-severe UC.
• Moderate-to-severe Crohn disease.

Advantages include:

• Gut-selective mechanism.
• Lower systemic immunologic effects than broadly systemic immunosuppressive agents.

Delayed onset relative to some small molecules should be considered when rapid disease control is required.

Ustekinumab

Ustekinumab targets the p40 subunit shared by IL-12 and IL-23.

It is approved for:

• Moderate-to-severe Crohn disease.
• Moderate-to-severe ulcerative colitis.

Maintenance therapy is administered subcutaneously after intravenous induction in Crohn disease and UC.

Selective IL-23 inhibitors

Selective IL-23 inhibitors target the p19 subunit of IL-23.

This class includes agents such as:

• Risankizumab.
• Guselkumab.
• Mirikizumab.

Their role has expanded following positive randomized trials in moderate-to-severe IBD.

Landmark biologic trials

Trial	Agent / strategy	Disease	Major contribution
ACCENT I	Infliximab maintenance	Crohn disease	Established scheduled infliximab maintenance
ACCENT II	Infliximab	Fistulizing Crohn disease	Demonstrated benefit in fistulizing disease
ACT 1 / ACT 2	Infliximab	UC	Established efficacy in moderate-to-severe UC
SONIC	Infliximab ± azathioprine	Crohn disease	Combination therapy improved outcomes compared with either agent alone
UC-SUCCESS	Infliximab ± azathioprine	UC	Supported combination therapy in selected biologic-naive patients
GEMINI 1	Vedolizumab	UC	Established induction and maintenance efficacy
GEMINI 2	Vedolizumab	Crohn disease	Established induction and maintenance efficacy
UNITI / IM-UNITI	Ustekinumab	Crohn disease	Established induction and maintenance efficacy
UNIFI	Ustekinumab	UC	Established induction and maintenance efficacy
VARSITY	Vedolizumab versus adalimumab	UC	Head-to-head evidence for biologic selection

Small-molecule therapies

Small molecules provide oral targeted treatment and may have rapid onset.

Major groups include:

• Janus kinase (JAK) inhibitors.
• Sphingosine-1-phosphate (S1P) receptor modulators.

JAK inhibitors

JAK inhibitors used in IBD include:

• Tofacitinib.
• Upadacitinib.

Advantages:

• Oral administration.
• Rapid onset of action.
• No anti-drug antibodies.
• Useful after biologic exposure in selected patients.

Important risks include:

• Serious infections.
• Herpes zoster.
• Malignancy.
• Major adverse cardiovascular events.
• Venous thromboembolism.

Risk assessment should consider:

• Age.
• Cardiovascular disease.
• Smoking.
• Previous thromboembolism.
• Malignancy risk.
• Infection risk.

S1P receptor modulators

Ozanimod and etrasimod are oral S1P receptor modulators used for ulcerative colitis.

Important considerations include:

• Bradycardia or conduction abnormalities during initiation in selected patients.
• Macular edema risk.
• Liver test abnormalities.
• Lymphocyte reduction.
• Infection risk.
• Drug interactions.

Pivotal small-molecule trials

Trial	Agent	Disease	Major contribution
OCTAVE	Tofacitinib	UC	Established induction and maintenance efficacy
U-ACHIEVE / U-ACCOMPLISH	Upadacitinib	UC	Established induction efficacy
U-ACHIEVE maintenance	Upadacitinib	UC	Established maintenance efficacy
U-EXCEL / U-EXCEED	Upadacitinib	Crohn disease	Established induction efficacy
U-ENDURE	Upadacitinib	Crohn disease	Established maintenance efficacy
True North	Ozanimod	UC	Established induction and maintenance efficacy
ELEVATE UC	Etrasimod	UC	Established induction and maintenance efficacy

IL-23 and newer advanced-therapy evidence

Trial	Agent	Disease
ADVANCE / MOTIVATE	Risankizumab	Crohn disease
FORTIFY	Risankizumab	Crohn disease maintenance
SEQUENCE	Risankizumab versus ustekinumab	Crohn disease
LUCENT	Mirikizumab	UC
QUASAR	Guselkumab	UC
GRAVITI	Guselkumab	Crohn disease

Therapeutic positioning

Treatment selection should incorporate:

• Disease phenotype.
• Disease severity.
• Risk of progression.
• Prior biologic exposure.
• Primary nonresponse versus secondary loss of response.
• Perianal disease.
• Extraintestinal manifestations.
• Infection risk.
• Malignancy risk.
• Cardiovascular and thrombotic risk.
• Pregnancy considerations.
• Patient preference.

Clinical situation	Treatment considerations
Mild UC	5-ASA-based therapy when appropriate
Moderate-to-severe UC	Advanced therapy or targeted therapy according to disease characteristics and prior exposure
Moderate-to-severe Crohn disease	Advanced therapy; corticosteroids may be used for induction but should not be maintenance
Fistulizing Crohn disease	Anti-TNF therapy, particularly infliximab, plus surgical management when required
High-risk Crohn disease	Consider early effective advanced therapy rather than prolonged ineffective step-up therapy
Prior anti-TNF failure	Consider another mechanism or optimized anti-TNF therapy depending on whether failure reflects underexposure or mechanistic failure
High infection risk	Consider mechanism-specific safety profile and minimize corticosteroids
High VTE/cardiovascular risk	Exercise particular caution with JAK inhibitors

Early effective therapy and risk stratification

Important high-risk Crohn disease features include:

• Young age at diagnosis.
• Extensive small-bowel disease.
• Deep ulcerations.
• Perianal disease.
• Stricturing phenotype.
• Penetrating phenotype.
• Repeated corticosteroid requirement.
• Previous surgery.
• Severe endoscopic or radiologic inflammation.

Patients with mild, nonprogressive disease may not require immediate aggressive escalation.

Early combination therapy evidence

SONIC trial

SONIC compared infliximab monotherapy, azathioprine monotherapy, and combination infliximab plus azathioprine in biologic- and immunomodulator-naive Crohn disease.

Combination therapy produced higher rates of corticosteroid-free clinical remission and mucosal healing than either monotherapy strategy in the studied population.

UC-SUCCESS trial

UC-SUCCESS evaluated infliximab monotherapy, azathioprine monotherapy, and combination therapy in patients with moderate-to-severe UC.

Combination therapy improved steroid-free clinical remission compared with either monotherapy in the studied population.

PROFILE trial

PROFILE evaluated early infliximab plus immunomodulator therapy compared with conventional step-up management in Crohn disease.

Reported outcomes included:

• Symptomatic remission at 1 year: RR 1.12; 95% CI 0.98–1.30.
• Steroid-free, surgery-free remission: 79% versus 15%.

The 79% versus 15% result should not be described as the primary symptomatic remission endpoint.

REACT-1 trial

REACT-1 evaluated an accelerated treatment strategy in Crohn disease.

Important findings included:

• No significant difference in steroid-free remission.
• Reduction in disease-related adverse events:
  • HR 0.73.
  • 95% CI 0.62–0.86.

Combination therapy

Biologic plus immunomodulator therapy may:

• Increase efficacy in selected patients.
• Reduce anti-drug antibody formation.
• Improve pharmacokinetic exposure.

Combination therapy should be individualized because immunomodulators increase certain infection and malignancy risks.

Loss of response and treatment optimization

Secondary loss of response should prompt objective assessment before empiric switching.

Evaluate:

• Symptoms.
• CRP.
• Fecal calprotectin.
• Stool infection testing when appropriate.
• Endoscopy.
• Cross-sectional imaging.
• Therapeutic drug levels when appropriate.

If inflammation is confirmed, determine whether treatment failure represents:

• Underexposure.
• Immunogenicity.
• Mechanistic failure.

Therapeutic drug monitoring

Therapeutic drug monitoring (TDM) measures serum drug concentrations, usually trough concentrations, with assessment of antidrug antibodies when relevant.

TDM is most established for TNF antagonists.

Reactive TDM

Reactive TDM is performed when objective evidence suggests:

• Primary nonresponse.
• Secondary loss of response.
• Biochemical activity.
• Endoscopic activity.
• Radiologic inflammation.

Drug concentration	Antidrug antibodies	Interpretation	General response
Low	Absent or low titer	Underexposure	Dose intensification or interval shortening
Low	High titer	Immunogenic failure	Consider switching therapy; selected patients may benefit from an immunomodulator
Adequate	Any	Possible mechanistic failure	Consider switching mechanism after confirming active inflammation

Infliximab and adalimumab exposure

In patients with active inflammation and suspected anti-TNF failure, adequate drug exposure should be established before labeling the mechanism ineffective.

Trough concentrations around 10–15 µg/mL are commonly used as a practical exposure threshold in patients with active disease when evaluating secondary loss of response, although target concentrations vary according to disease state, assay, indication, and desired outcome.

Thiopurine monitoring

A commonly cited therapeutic range for 6-thioguanine nucleotides is approximately:

• 230–450 pmol/8 × 10⁸ red blood cells.

Interpretation should incorporate leukocyte count, liver tests, clinical response, and toxicity.

Proactive TDM

Proactive TDM involves scheduled drug-concentration measurement in patients without apparent loss of response.

Evidence remains mixed.

Special Situations

Certain IBD presentations require disease-specific management strategies because of their severity, complications, or unique treatment considerations.

Acute severe ulcerative colitis

Acute severe ulcerative colitis (ASUC) is a medical emergency requiring hospitalization, prompt assessment, intravenous corticosteroids, VTE prophylaxis unless contraindicated, close monitoring, and early colorectal surgical involvement.

Initial evaluation

Initial assessment should include:

• Stool frequency.
• Rectal bleeding.
• Abdominal pain.
• Fever.
• Heart rate and blood pressure.
• Abdominal examination.

Laboratory evaluation:

• CBC.
• CRP.
• Electrolytes.
• Albumin.
• Renal function.

Additional evaluation:

• Stool testing for infectious causes, particularly Clostridioides difficile.
• Flexible sigmoidoscopy with biopsies when appropriate.
• Assessment for CMV in severe or steroid-refractory disease.

Initial management

Management includes:

• Intravenous corticosteroids.
• Pharmacologic VTE prophylaxis unless contraindicated.
• Appropriate infection evaluation.
• Enteral nutrition when tolerated.
• Early colorectal surgical consultation.

Corticosteroid induction

Common regimens include:

• Methylprednisolone approximately 40–60 mg/day intravenously.
• Hydrocortisone 100 mg intravenously three to four times daily.

Clinical response should be assessed after approximately 3 days.

Predicting corticosteroid failure

Oxford/Travis criteria identify patients at increased risk of colectomy.

High-risk criteria include:

• Stool frequency >8 bowel movements/day.

OR

• Stool frequency 3–8 bowel movements/day with CRP >45 mg/L.

Rescue therapy

For corticosteroid-refractory ASUC, established rescue options include:

• Infliximab.
• Cyclosporine.

Selection should consider:

• Prior biologic exposure.
• Previous infliximab use.
• Comorbidities.
• Renal function.
• Infection risk.
• Long-term maintenance strategy.

CMV colitis

CMV reactivation may occur in severely inflamed or corticosteroid-refractory colitis.

Important principles:

• CMV should be considered in severe or steroid-refractory disease.
• Biopsies from ulcer bases may improve diagnostic yield.
• Immunohistochemistry and tissue-based testing are commonly used.
• Antiviral treatment is generally reserved for clinically significant CMV infection.

Treatment may include:

• Intravenous ganciclovir followed by oral therapy.
• Valganciclovir in selected patients.

Surgical management

Urgent colectomy is indicated for:

• Perforation.
• Toxic megacolon.
• Massive hemorrhage.
• Progressive clinical deterioration.
• Failure of appropriate medical rescue therapy.

Venous thromboembolism prevention

Patients with IBD have increased risk of venous thromboembolism, particularly during:

• Active inflammation.
• Hospitalization.
• Severe disease.
• Corticosteroid exposure.
• Recent surgery.
• JAK inhibitor exposure.

Pharmacologic thromboprophylaxis should generally be provided during hospitalization for active IBD unless contraindicated.

Inflammatory rectal bleeding alone is not generally considered a reason to withhold pharmacologic prophylaxis.

Individualize prophylaxis in:

• Hemodynamic instability.
• Massive hemorrhage.
• Severe thrombocytopenia.
• Imminent invasive procedures.
• Other major bleeding risks.

Perianal Crohn disease

Perianal Crohn disease requires combined medical and surgical management.

Assessment may include:

• Pelvic MRI.
• Examination under anesthesia.
• Surgical consultation.

An abscess should be drained before escalation of immunosuppression.

Seton placement may be appropriate for selected fistulas.

Anti-TNF therapy, particularly infliximab, has strong evidence for fistulizing Crohn disease.

Preventive care and vaccination

Preventive care should be individualized according to:

• Age.
• Disease activity.
• Medication exposure.
• Immunosuppression.
• Comorbidities.
• Pregnancy plans.

Vaccination

• Inactivated vaccines are generally safe in IBD.
• Live vaccines should generally be avoided during significant immunosuppression.
• Vaccination should ideally be completed before immune-modifying therapy when feasible.

Recommended vaccines include:

• Annual influenza.
• COVID-19.
• Pneumococcal vaccination.
• Recombinant zoster vaccine.
• Hepatitis B when nonimmune.
• HPV according to age and eligibility.

Infection screening

Before selected biologic or targeted therapies, evaluate for:

• Tuberculosis.
• Hepatitis B.
• Hepatitis C when clinically indicated.
• HIV when clinically indicated.

Colorectal cancer surveillance

Patients with longstanding colonic IBD have increased colorectal cancer risk.

Surveillance applies primarily to:

• Ulcerative colitis extending beyond the rectum.
• Crohn colitis with substantial colonic involvement.

Routine surveillance generally begins approximately 8 years after onset of substantial colonic disease.

Patients with primary sclerosing cholangitis should begin surveillance at the time PSC is diagnosed regardless of IBD duration.

Preferred approaches include:

• High-definition colonoscopy.
• Chromoendoscopy when available.
• Targeted biopsies of visible lesions.

Surveillance should ideally be performed during clinical remission.

Pregnancy

Patients with IBD should ideally conceive while disease is in documented remission.

Many therapies can be continued when clinically indicated, including:

• 5-ASA agents.
• Sulfasalazine.
• Thiopurines.
• Anti-TNF therapy.
• Vedolizumab.
• Ustekinumab.

Generally avoid:

• Methotrexate.
• JAK inhibitors.
• S1P receptor modulators.

Sulfasalazine should be accompanied by folic acid supplementation.

Agent-specific recommendations should follow current prescribing information and reproductive guidance.

De-escalation and withdrawal

Treatment de-escalation should be considered only after sustained objective disease control and should be individualized.

Before withdrawal or major dose reduction, assess:

• Clinical remission.
• CRP.
• Fecal calprotectin.
• Endoscopic activity.
• Disease phenotype.
• Previous relapse history.
• Duration of remission.
• Prior complications.

Patients with previous penetrating, stricturing, perianal, or severe disease may have substantial relapse risk after treatment withdrawal.

Corticosteroids should be withdrawn whenever possible.

Common treatment pitfalls

• Treating symptoms without confirming objective inflammatory control.
• Using corticosteroids as maintenance therapy.
• Delaying steroid-sparing therapy in steroid-dependent disease.
• Treating Crohn disease with mesalamine when advanced therapy is indicated.
• Presenting PROFILE secondary outcomes as the primary clinical-remission endpoint.
• Describing REACT-2 as an early-combination-therapy trial.
• Applying STARDUST every-12-week ustekinumab dosing as the standard U.S. maintenance regimen.
• Escalating therapy without confirming objective inflammation.
• Switching biologics without distinguishing pharmacokinetic failure from mechanistic failure.
• Withholding pharmacologic VTE prophylaxis solely because of inflammatory rectal bleeding.
• Delaying surgical consultation in ASUC.
• Failing to screen for tuberculosis and hepatitis B before selected advanced therapies.

High-yield treatment summary

Clinical problem	High-yield approach
Mild UC	5-ASA-based therapy when appropriate
Moderate-to-severe UC	Advanced biologic or targeted therapy based on disease characteristics and prior exposure
Moderate-to-severe Crohn disease	Advanced therapy; corticosteroids only for induction when needed
Steroid dependence	Initiate or optimize steroid-sparing therapy and discontinue corticosteroids
Fistulizing Crohn disease	Drain abscesses, coordinate surgical care, and use effective biologic therapy such as anti-TNF treatment
Persistent symptoms on biologic therapy	Confirm objective inflammation and evaluate for infection, pharmacokinetic failure, or mechanistic failure
Secondary loss of response	Consider reactive TDM for appropriate agents
ASUC	Hospitalize, IV corticosteroids, VTE prophylaxis, reassess early, rescue therapy or surgery when indicated
High VTE risk	Control inflammation, minimize steroids, provide inpatient prophylaxis unless contraindicated
Long-term management	Treat to objective targets and maintain steroid-free remission

Complications , Natural history and Surgery

Major complications

Crohn disease	Ulcerative colitis
Stricture	Toxic megacolon
Small-bowel obstruction	Severe hemorrhage
Fistula	Colorectal dysplasia and cancer
Abscess	Primary sclerosing cholangitis
Perianal disease	Colectomy-related complications
Malabsorption and nutritional deficiencies	Venous thromboembolism
Nephrolithiasis	Osteoporosis
Short bowel syndrome after extensive resection	Extraintestinal manifestations

Contemporary surgical rates are substantially lower than historical pre-biologic estimates.

Time after diagnosis	Ulcerative colitis	Crohn disease
1 year	~2.8%	~12.3%
5 years	~7.0%	~18.0%
10 years	~9.6%	~26.2%

The above estimates represent modern all-era pooled estimates rather than historical cohorts.

Earlier pooled estimates across all treatment eras reported:

• Ulcerative colitis:
  • 4.0% at 1 year.
  • 8.8% at 5 years.
  • 13.3% at 10 years.

• Crohn disease:
  • 18.7% at 1 year.
  • 28.0% at 5 years.
  • 39.5% at 10 years.

A genuinely pre-biologic estimate reported approximately 46.6% cumulative Crohn disease surgery by 10 years.

Declining surgery rates likely reflect multiple factors, including:

• Earlier diagnosis.
• Improved monitoring.
• Better multidisciplinary care.
• Modern medical therapy.
• Changes in surgical practice.

Surgery is not curative for Crohn disease.

Following first intestinal resection:

• Approximately 17.7% require a second resection within 5 years.
• Approximately 31.3% require a second resection within 10 years.

Postoperative Crohn disease

Risk factors for postoperative recurrence include:

• Active smoking.
• Penetrating disease.
• Prior intestinal resection.
• Extensive small bowel disease.
• Perianal disease.

Low-risk patients

Characteristics:

• Nonsmoker.
• No penetrating phenotype.
• No previous resection.

Management:

• Observation may be appropriate.
• Ileocolonoscopy approximately 6 months after surgery.

Intermediate-risk patients

Management options:

• Thiopurine therapy.
• Thiopurine plus short-course metronidazole.

Escalate therapy when Rutgeerts score is ≥i2b.

High-risk patients

Examples:

• Previous resection within 10 years.
• Multiple prior resections.
• Active smoking.

Preferred therapy:

• Anti-TNF therapy.
• Consider combination immunomodulator.

Repeat ileocolonoscopy approximately 6 months postoperatively.

Rutgeerts score

Score	Findings
i0	No lesions
i1	≤5 aphthous ulcers
i2	>5 aphthous ulcers with normal intervening mucosa or lesions confined to the anastomosis
i3	Diffuse aphthous ileitis
i4	Diffuse inflammation with large ulcers, severe lesions, or narrowing

Key evidence

POCER

• Colonoscopy-guided treatment escalation reduced postoperative endoscopic recurrence.

REPREVIO

• Early postoperative vedolizumab reduced severe endoscopic recurrence compared with placebo.

Mesalamine is not recommended for postoperative prophylaxis.

Pouchitis and inflammatory pouch disorders

Inflammatory disorders of the ileal pouch occur after restorative proctocolectomy with ileal pouch-anal anastomosis (IPAA).

Diagnosis requires:

• Endoscopic confirmation of inflammation.
• Exclusion of alternative causes.

AGA guidance consists primarily of conditional recommendations based on low-certainty evidence.

Management

Intermittent pouchitis

Management:

• Antibiotics.

Recurrent antibiotic-responsive pouchitis

Management:

• Consider probiotics to reduce recurrence.

Chronic antibiotic-dependent pouchitis

Options include:

• Chronic antibiotics.
• Advanced therapies when long-term antibiotics are undesirable or ineffective.

Chronic antibiotic-refractory pouchitis

Management:

• Advanced immunosuppressive therapy.
• Corticosteroids may be considered in selected patients.

Crohn-like disease of the pouch

Management:

• Corticosteroids.
• Advanced biologic or targeted therapy.

Cuffitis

Initial treatment:

• Topical 5-ASA.
• Topical corticosteroids.

Current evidence has not demonstrated clear superiority of one advanced therapy class over another.

Prognosis

These factors are associations rather than absolute predictors of an individual patient's disease course.

Prognostic factors

Disease	Factors associated with more complicated disease course	Factors associated with more favorable course
Crohn disease	Young age at diagnosis Extensive small-bowel disease Deep ulcerations Stricturing or penetrating disease Perianal disease Early corticosteroid requirement Smoking	Limited inflammatory disease Absence of stricturing or penetrating complications Sustained objective remission
Ulcerative colitis	Extensive colitis; Severe endoscopic activity Frequent flares Corticosteroid dependence Primary sclerosing cholangitis	Limited disease extent Sustained clinical and endoscopic remission Steroid-free disease control
Both	Persistent objective inflammation Repeated hospitalization Poor treatment response Significant comorbidity	Good treatment response; Sustained steroid-free remission Good adherence to maintenance therapy

These factors are associations rather than absolute predictors of an individual patient's disease course. Prognosis should be assessed according to disease phenotype, inflammatory burden, complications, treatment response, and comorbidities.

Screening

Assessment should include:

• Complete blood count.
• Ferritin.
• Transferrin saturation.
• C-reactive protein (CRP).

Monitoring frequency:

• Every 6–12 months during remission or mild disease.
• Approximately every 3 months during active disease.

Ferritin is an acute-phase reactant; therefore, normal or elevated ferritin does not exclude iron deficiency during active inflammation.

Additional nutritional assessment should include:

• Vitamin B12.
• Folate.
• Vitamin D.
• Other micronutrients when clinically indicated.

Iron replacement

Treatment principles:

• Optimize control of intestinal inflammation.
• Correct iron deficiency in all patients with iron deficiency anemia.

Intravenous iron is preferred in:

• Active IBD.
• Hemoglobin <10 g/dL.
• Significant anemia.
• Oral iron intolerance.
• Failure of oral iron therapy.
• Extensive intestinal disease.

Randomized trials demonstrate that intravenous iron is associated with:

• Greater likelihood of hemoglobin increase ≥2 g/dL.
• Lower treatment discontinuation rates than oral iron.

ECCO recommends intravenous iron as first-line therapy for active disease and clinically significant anemia.

Vitamin B12 deficiency

Risk factors include:

• Terminal ileal Crohn disease.
• Ileal resection.
• Extensive ileal involvement.

Management:

• Screen patients with ileal disease or previous ileal resection.
• Replace vitamin B12 when deficiency is identified.

Folate deficiency

Risk factors:

• Poor nutritional intake.
• Malabsorption.
• Methotrexate therapy.

Management:

• Replace folate deficiency.
• Supplement when clinically indicated.

Vitamin D deficiency and bone health

Vitamin D deficiency is particularly common in Crohn disease.

Potential consequences:

• Reduced bone mineral density.
• Osteopenia.
• Osteoporosis.
• Increased fracture risk.

Management:

• Assess vitamin D status in at-risk patients.
• Replace deficiency.
• Minimize corticosteroid exposure whenever possible.

Perianal Crohn disease

Perianal Crohn disease represents an aggressive phenotype associated with significant morbidity.

Evaluation should include:

• Pelvic MRI.
• Examination under anesthesia when appropriate.

Abscess management

Drainage of abscess is mandatory before initiation of immunosuppressive therapy.

Management may include:

• Surgical drainage.
• Seton placement for complex fistulas.

Medical therapy

Anti-TNF therapy remains the best-established biologic treatment.

Infliximab has the strongest evidence for:

• Fistula closure.
• Durable fistula healing.

Persistent fistulizing disease requires multidisciplinary management involving:

• Gastroenterology.
• Colorectal surgery.
• Radiology.

Therapeutic drug monitoring

Therapeutic drug monitoring (TDM) measures serum drug concentration—usually trough concentration—together with antidrug antibodies.

It is most established for TNFα antagonists.

Rationale

Exposure–response relationship

Across ACT, PURSUIT, GEMINI, UNIFI, FORTIFY, PREVENT, ADVANCE, and MOTIVATE studies:

• Patients in the lowest drug-concentration quartile achieved clinical remission and endoscopic outcomes nearly identical to placebo.

Variability in clearance

Drug clearance varies because of:

• Immune-mediated mechanisms:
  • Neutralizing antidrug antibodies.

• Nonimmune-mediated mechanisms:
  • High inflammatory burden.
  • Protein loss.

Mechanistic failure

Adequate drug exposure may still fail when the targeted inflammatory pathway is not the principal driver of disease.

Reactive TDM

Reactive TDM is performed in patients with:

• Clinical evidence of inflammation.
• Biochemical activity.
• Radiologic inflammation.
• Endoscopic inflammation.

It is supported for:

• Primary nonresponse.
• Secondary loss of response.

Drug concentration	Antidrug antibodies	Interpretation	Management
Low	Absent or low titer	Underexposure	Dose intensification or interval shortening
Low	High titer	Immunogenic failure	Switch therapy; consider adding immunomodulator
Adequate	Any	Mechanistic failure	Switch mechanism of action

Concentration threshold before declaring failure

Infliximab or adalimumab should not be considered mechanistically ineffective until trough concentrations of approximately:

10–15 µg/mL

have been achieved.

Stopping therapy below this threshold risks misclassifying underdosing as treatment failure.

Thiopurine monitoring

Target:

• 6-thioguanine nucleotide concentration of 230–450 pmol/8 × 10⁸ red blood cells.

Proactive TDM — unresolved

Proactive TDM involves routine measurement regardless of disease activity.

Current evidence remains conflicting.

Evidence against routine use:

• Meta-analysis of 9 randomized trials found no improvement in clinical remission.
• Increased therapy escalation occurred without reduction in antidrug antibody formation.

Evidence supporting proactive monitoring:

• Meta-analysis of 8 studies found reduced treatment failure, although evidence was mainly observational.

Expert consensus:

• A modified Delphi panel supported proactive anti-TNF monitoring after induction and at least once during maintenance.

Current society position:

• AGA and ECCO consider proactive TDM a knowledge gap and make no recommendation for or against routine use.

Practical principles

• Reactive TDM is established practice for loss of response and primary nonresponse.
• Proactive TDM should be individualized.
• Do not abandon infliximab or adalimumab below 10–15 µg/mL trough concentrations.
• Drug concentrations should always be interpreted with symptoms, CRP, fecal calprotectin, endoscopy, and imaging.

Common pitfalls

• Treating symptoms without confirming objective inflammatory control.
• Using corticosteroids as maintenance therapy.
• Presenting PROFILE secondary outcomes as the primary remission outcome.
• Describing REACT-2 as an early-combination therapy trial.
• Applying STARDUST every-12-week ustekinumab dosing to U.S. clinical practice.
• Applying all-era pooled surgery estimates as historical pre-biologic cohorts.
• Describing filgotinib as FDA-approved.
• Withholding pharmacologic VTE prophylaxis solely because of inflammatory rectal bleeding.
• Omitting recombinant zoster vaccination before immune-modifying therapy when indicated.
• Delaying colectomy solely to complete antiviral therapy in corticosteroid-refractory CMV colitis.

Differential diagnosis

The differential diagnosis of inflammatory bowel disease (IBD) includes conditions that mimic IBD and conditions that superimpose on established IBD. Because chronic diarrhea, abdominal pain, and rectal bleeding are nonspecific, distinction requires integration of clinical profiling, biomarkers, endoscopy with biopsy, cross-sectional imaging, and microbiology; no single test is definitive.

Practical framework

The major categories of IBD mimics include infectious, drug-induced, vascular, structural, radiation-associated, diversion-related, immune-mediated, and functional disorders. Accurate patient profiling should direct the differential, including age, geography and tuberculosis exposure, immune status, medication history, vascular risk, and relevant sexual history.

Differential category	High-yield examples	Key discriminator
Functional	IBS	No objective intestinal inflammation; consider fecal calprotectin and alarm features
Infectious	Salmonella, Shigella, Campylobacter, Yersinia, diarrheagenic E. coli, Entamoeba histolytica, C. difficile, CMV	Stool testing, exposure history, microbiology, and compatible distribution
Mycobacterial	Intestinal tuberculosis	Epidemiologic risk, necrotic lymphadenopathy, tissue microbiology/histology
Vascular	Ischemic colitis	Acute onset, vascular risk factors, watershed distribution
Structural	SCAD, diverticular disease	Segmental diverticular distribution and rectal sparing
Iatrogenic	NSAID enteropathy/colitis, checkpoint-inhibitor colitis, mycophenolate-associated colitis, radiation colitis, diversion proctocolitis	Medication, treatment, or surgical history
Immune/inflammatory	Behçet disease, celiac disease, eosinophilic gastrointestinal disease	Extraintestinal manifestations and characteristic histology/distribution
Other	Microscopic colitis, sexually transmitted proctitis, monogenic/VEO-IBD	Histology, sexual/travel history, age at onset, and targeted testing

IBD versus irritable bowel syndrome

IBS is a disorder of gut-brain interaction diagnosed using Rome IV criteria and does not cause the objective mucosal inflammation or chronic architectural changes characteristic of IBD.

Fecal calprotectin (FC) is a high-yield noninvasive discriminator between inflammatory and functional bowel disease. At a threshold of ≥50 μg/g, pooled sensitivity was approximately 85.8% and specificity approximately 91.7% in a systematic review; performance varies according to threshold, population, and study setting.

CRP is supportive but nonspecific. A proportion of patients with active intestinal inflammation have a limited CRP response, so a normal CRP does not exclude active IBD.

Alarm features such as weight loss, gastrointestinal bleeding, nocturnal diarrhea, anemia, family history of IBD or colorectal cancer, and new symptoms at older age should prompt evaluation for organic disease rather than a functional diagnosis.

Infectious colitis and enteritis

Infectious colitis can closely mimic an IBD flare. Important pathogens include Salmonella, Shigella, Campylobacter, Yersinia, diarrheagenic E. coli, Entamoeba histolytica, C. difficile, and CMV.

Infection	Distribution or clue that may mimic IBD
Salmonella / Yersinia	Terminal ileum and ileocecal region; can mimic Crohn's disease
Shigella	Predominantly left-sided colitis
E. histolytica	Cecal or patchy colitis
C. difficile / CMV	Can produce extensive colitis and mimic an IBD flare
Infection overall	Long-segment or continuous involvement and a self-limited course may favor infection

Stool testing should be obtained before attributing new symptoms to IBD, particularly during an apparent flare. Testing may include culture or multiplex PCR, C. difficile toxin/PCR, and ova and parasite testing when epidemiologically appropriate.

Intestinal tuberculosis versus Crohn's disease

Intestinal tuberculosis (ITB) is an important Crohn's disease mimic, particularly in patients from or traveling to endemic regions and in immunocompromised patients.

Both ITB and Crohn's disease may cause asymmetric ileocecal wall thickening, strictures, and abscesses. Findings that favor ITB include bulky necrotic lymphadenopathy; skip lesions, the comb sign, and perianal disease favor Crohn's disease.

Feature	Favors intestinal TB	Favors Crohn's disease
Lymph nodes	Bulky or necrotic lymphadenopathy	Usually less characteristic necrotic adenopathy
Bowel distribution	Ileocecal predominance	Segmental/skip involvement; ileocolonic disease common
Cross-sectional imaging	Necrotic nodes and compatible ileocecal disease	Comb sign and characteristic penetrating/inflammatory complications
Perianal disease	Less characteristic	Supports Crohn's disease
Tissue findings	Caseating granulomas or positive AFB/culture/PCR may support TB	Noncaseating granulomatous inflammation may support Crohn's disease but is not required

When ITB is plausible, targeted evaluation may include IGRA and tissue AFB stain, culture, and/or PCR, together with histopathologic assessment. Immunosuppression for presumed Crohn's disease should not be initiated without adequately addressing the possibility of active ITB.

Superimposed infection in established IBD

In a patient with established IBD, worsening symptoms may represent active IBD, infection, or both. C. difficile and CMV are particularly important superimposed infections. C. difficile is more common in IBD and is associated with worse clinical outcomes; concurrent CMV and C. difficile infection may further worsen prognosis.

At an apparent flare, particularly in hospitalized or immunosuppressed patients, evaluate for infectious causes rather than assuming that increased symptoms represent idiopathic inflammatory activity. CMV testing is particularly relevant in severe or steroid-refractory colitis.

Vascular and structural mimics

Condition	Distinguishing features
Ischemic colitis	Older age or vascular risk factors; acute onset; watershed distribution, particularly around the splenic flexure; may show thumbprinting on imaging
SCAD	Inflammation confined to a diverticular segment, usually sigmoid, with rectal sparing; can mimic Crohn's colitis
Radiation colitis/proctitis	History of prior pelvic or prostate radiotherapy
Diversion proctocolitis	Occurs in defunctioned bowel after diversion

Drug-induced and iatrogenic colitis

Medication and treatment history is essential because several drug-induced disorders can mimic IBD clinically, endoscopically, or histologically.

Cause	Typical clue
NSAID enteropathy/colitis	Ulceration, strictures, and diaphragm-like webs; exposure history is critical
Immune checkpoint-inhibitor colitis	Temporal relationship to checkpoint-inhibitor therapy; can resemble IBD
Mycophenolate-associated colitis	Medication exposure with overlapping inflammatory histology
Radiation colitis/proctitis	Prior pelvic radiotherapy
Diversion proctocolitis	Defunctioned bowel after diversion

NSAIDs, PPIs, and SSRIs have also been associated with microscopic colitis.

Microscopic colitis

Microscopic colitis causes chronic watery, non-bloody diarrhea with normal or near-normal-appearing colonic mucosa. Diagnosis is histologic and requires colonic biopsies even when endoscopy appears normal.

Subtype	Histologic pattern
Lymphocytic colitis	≥20 intraepithelial lymphocytes per 100 epithelial cells without a thickened collagen band
Collagenous colitis	Thickened subepithelial collagen band (>10 μm) with increased intraepithelial lymphocytes
Incomplete microscopic colitis	Intermediate histologic abnormalities that do not meet classic thresholds, such as >5 intraepithelial lymphocytes per 100 epithelial cells or collagen >5 μm

Microscopic colitis is related to the broader spectrum of inflammatory colitides but is not a subtype of Crohn's disease or ulcerative colitis. Random biopsies of normal-appearing colon are required when clinical suspicion is present.

Other immune and inflammatory mimics

Condition	Distinguishing features
Behçet disease	Discrete, often large punched-out ileocecal ulcers with oral/genital ulcers and/or uveitis; can closely resemble Crohn's disease
Celiac disease	Diarrhea-predominant presentation; evaluate with appropriate celiac serology such as tTG-IgA
Eosinophilic gastrointestinal disease	Prominent mucosal eosinophilia without the characteristic IBD pattern; diagnosis requires clinicopathologic correlation and exclusion of secondary causes
Sexually transmitted proctitis	Consider Chlamydia trachomatis including LGV, gonorrhea, herpes simplex virus, and syphilis in appropriate sexual exposures; may mimic ulcerative proctitis
Monogenic/VEO-IBD	Consider in refractory very-early-onset disease, particularly when phenotype or associated immune abnormalities suggest an underlying genetic disorder

Clinically actionable approach

1. Evaluate for objective inflammation. In chronic diarrhea or abdominal pain without an established diagnosis, fecal calprotectin and CRP can help triage inflammatory from functional disease. A low fecal calprotectin substantially lowers the probability of active IBD, but interpretation depends on the clinical setting and threshold used.
2. Check for infection. Obtain appropriate stool infectious studies, including C. difficile testing and ova/parasite testing when indicated, before attributing symptoms to an IBD flare.
3. Consider intestinal TB before immunosuppression. In patients with epidemiologic risk or compatible ileocecal imaging, investigate ITB with targeted microbiologic and histologic testing.
4. Review medications and exposures. Ask specifically about NSAIDs, immune checkpoint inhibitors, mycophenolate, pelvic radiation, diversion surgery, travel, vascular risk, and relevant sexual exposures.
5. Use endoscopy with biopsy when indicated. Histology is essential for distinguishing several mimics and for diagnosing microscopic colitis; normal-appearing mucosa does not exclude microscopic disease.
6. Use cross-sectional imaging when small-bowel or transmural disease is suspected. Distribution, lymphadenopathy, penetrating complications, and other imaging patterns can help distinguish IBD from infection and other mimics.
7. Do not assume residual symptoms in known IBD are active inflammation or "just IBS." Objective assessment for inflammatory activity and organic causes should precede escalation or attribution of persistent symptoms to a functional disorder.

Important updates

• Fecal calprotectin has become a central first-line noninvasive test for distinguishing inflammatory from functional bowel symptoms, reducing reliance on nonspecific serologic markers.
• Cross-sectional imaging and intestinal ultrasound increasingly provide pattern-based information that complements endoscopy and biopsy, particularly for small-bowel and transmural disease.
• Infectious superinfection must remain in the differential of an apparent IBD flare, particularly C. difficile and CMV.
• IBD-IBS overlap is recognized as a distinct clinical problem. Functional symptoms can persist despite objective inflammatory remission and should not automatically trigger escalation of IBD-directed therapy.
• Commercial or experimental molecular/serologic classifiers may distinguish IBD phenotypes in research settings, but they do not replace standard clinical, endoscopic, histologic, microbiologic, and imaging assessment.

Areas of uncertainty

• Fecal calprotectin has no single universal cutoff. Thresholds vary by clinical purpose and population, and diagnostic performance is heterogeneous across studies.
• Crohn's disease versus intestinal TB remains difficult in regions where both conditions are encountered, and no single test reliably resolves every case.
• CMV detection in IBD requires clinical interpretation. Distinguishing clinically important CMV colitis from incidental mucosal detection can be challenging, particularly in severe or steroid-refractory disease.
• Crohn's colitis versus SCAD may remain difficult on biopsy alone when inflammation is segmental and associated with diverticular disease.
• Molecular and serologic classifiers remain investigational and are not substitutes for standard clinicopathologic assessment.

High-yield clinical pearls

• Low fecal calprotectin substantially lowers the likelihood of active IBD in an appropriate low-risk clinical setting.
• Bulky necrotic ileocecal lymphadenopathy should raise concern for intestinal TB rather than being attributed automatically to Crohn's disease.
• An apparent IBD flare should trigger consideration of infection, particularly C. difficile and CMV.
• Rectal sparing with segmental sigmoid inflammation suggests SCAD rather than classic UC.
• Watery non-bloody diarrhea with normal-appearing colonic mucosa should prompt biopsies for microscopic colitis.
• Large punched-out ileocecal ulcers with oral or genital ulcers should prompt consideration of intestinal Behçet disease.

Common pitfalls

• Diagnosing IBS without considering fecal calprotectin and alarm features in patients with possible organic disease.
• Attributing acute deterioration in established IBD to an inflammatory flare without appropriate stool infectious testing.
• Starting immunosuppression for presumed Crohn's disease without adequately considering intestinal TB in patients with compatible epidemiologic or imaging features.
• Overlooking NSAID, checkpoint-inhibitor, mycophenolate, radiation, or diversion-associated colitis because medication and procedural history was not obtained.
• Failing to biopsy normal-appearing colonic mucosa when microscopic colitis is clinically suspected.
• Labeling persistent symptoms in quiescent IBD as functional without objectively assessing inflammatory activity and other organic causes.

References

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