Pneumonia differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Hamid Qazi, MD, BSc [2], Priyamvada Singh, M.D. [3] Syed Hassan A. Kazmi BSc, MD [4]


Pneumonia should be differentiated from other conditions that cause cough, fever, shortness of breath and tachypnea, such as asthma, COPD, CHF, cancer, GERD, pulmonary emboli.

Differentiating Pneumonia from other Diseases

Differential Diagnosis of Pneumonia [1][2][3]
Disease Findings
Acute bronchitis No infiltrates seen on the chest X-ray.
Asthma Past medical history, no infiltrates seen on chest X-ray.
Bronchiolitis obliterans Should be suspected in patients with pneumonia who do not respond to antibiotics treatment.
Congestive heart failure Bilateral pulmonary edema, shortness of breath.
COPD Past medical history, no infiltrates on chest X-ray, fever is uncommon.
Empyema CXR showing features of pleural effusion, inflammatory markers on thoracocentesis.
Endocarditis Finding of septic pulmonary emboli
Gastroesophageal reflux disease (GERD) Normal chest X-ray, symptoms are worse during night and associated with meals.
Lung abscess Chest X-ray shows signs of lung abscess.
Lung cancer Weight loss, clear sputum. CT scan and biopsy are helpful in ruling out malignancy.
Pertussis Productive cough for weeks, nasopharyngeal aspirate aids in diagnosis.
Pulmonary embolus A high degree of suspicion should be kept for pulmonary embolus. Chest X-ray may be normal.
Sinusitis Sinus tenderness, post nasal drip.
Vasculitis Systemic manifestations of collagen vascular disease may be seen.

Differential diagnosis

Causes of

Lung Cavities

Differentiating Features Differentiating Radiological Findings Diagnosis Confirmation
  • CXR and CT demonstrates cavities in the upper lobe of the lung
  • Sputum smear positive for acid-fast bacilli and nucleic acid amplification tests (NAAT) is used on sputum or any sterile fluid for rapid diagnosis and is positive for mycobacteria.
  • Any age group
  • Acute, fulminant life threating complication of prior infection
  • >100.4F fever, with hemodynamic instability
  • Worsening pneumonia-like symptoms
  • CBC is positive for causative organism
  • Children and elderly are at risk
  • Empyema appears lenticular in shape and has a thin wall with smooth luminal margins
  • Pulmonary nodules with cavities and infiltrates are a frequent manifestation on CXR
  • Elderly females of 40-50 age group
  • Manifestation of rheumatoid arthritis
  • Presents with other systemic symptoms including symmetric arthritis of the small joints of the hands and feet with morning stiffness are common manifestations.
  • Pulmonary nodules with cavitation are located in the upper lobe (Caplan syndrome) on Xray.
  • On CXR bilateral adenopathy and coarse reticular opacities are seen.
  • CT of the chest demonstrates extensive hilar and mediastinal adenopathy
  • Additional findings on CT include fibrosis (honeycomb, linear, or associated with bronchial distortion), pleural thickening, and ground-glass opacities.[10]
  • Common appearance on CT is patchy consolidation,often accompanied by ground-glass opacities and nodules.[14]
  • Exclusively afflicts smokers, with a peak age of onset of between 20 and 40 years.
  • Clinical presentation varies, but symptoms generally include months of dry cough, fever, night sweats and weight loss.
  • Skin is involved in 80% of the cases, scaly erythematous rash is typical.
  • Thin-walled cystic cavities are the usual radiographic manifestation, observed in over 50% of patients by either CXR or CT scans.[16]
  • Biopsy of the lung

Table 1: Differentiating psittacosis from other diseases

Clinical feature Cough Sputum Dyspnea Sore throat Headache Confusion Diarrhea Chest radiograph changes Hyponatremia Leukopenia Abnormal Liver function tests Treatment
Psittacosis ++ - + - +++ + Minimal
  • No changes seen
- + - Doxycycline
C.pneumoniae pneumonia + + + +++ ++ + -
  • Minimal changes observed
- - - Doxycycline, Azithromycin
M. pneumoniae pneumonia ++ ++ ++ - - - - - - + Doxycycline
L. Pneumophila infection + +++ +++ - + ++ + Often Multifocal ++ + ++ Doxycycline
Influenza ++ ++ ++ ++ ++ +/- +/- - - - zanamivir, oseltamivir,
Endocarditis ++ ++ + - - - -
  • Hazy opacities at lung

bases bilaterally

- +/- +/- Vancomycin
Coxiella burnetii infection ++ - + +/- - +/- Minimal - +/- =/- Doxycycline
Leptospirosis ++ + ++ + + ++ -
  • Multiple ill-defined nodules in both lungs.
+++ Doxycycline, azithromycin, amoxicillin
Brucellosis ++ - + - ++ + - -/+ +/- +/- Doxycycline, rifampin


+, occurs in some cases

++, occurs in many cases,

+++, occurs frequently

Pnemonia must be differentiated from other diseases that cause atypical pneumonia such as Q fever and legionella pneumonia:

Disease Prominent clinical features Lab findings Chest X-ray
Q fever
  • Antibody detection using indirect immunofluorescence (IIF) is the preferred method for diagnosis.
  • PCR can be used if IIF is negative, or very early once disease is suspected.
  • C. burnetii does not grow on ordinary blood cultures, but can be cultivated on special media such as embryonated eggs or cell culture.
  • A two-to-three fold increase in AST and ALT is seen in most patients.
Q fever pneumonia - - Case courtesy of Royal Melbourne Hospital Respiratory,, rID 21993
Mycoplasma pneumonia
Mycoplasma pneumonia - Case courtesy of Dr Alborz Jahangiri,, rID 45781
Legionella pneumonia - Case courtesy of Dr Henry Knipe,, rID 31816
Chlamydia pneumonia
Chlamydia-pneumonia - Case courtesy of Dr Andrew Dixon,, rID 14567

Other differentials

Pneumonia should be differentiated from other diseases presenting with cough, fever, shortness of breath and tachypnea. The differentials include the following:[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]

Diseases Diagnostic tests Physical Examination Symptoms Past medical history Other Findings
CT scan and MRI EKG Chest X-ray Tachypnea Tachycardia Fever Chest Pain Hemoptysis Dyspnea on Exertion Wheezing Chest Tenderness Nasalopharyngeal Ulceration Carotid Bruit
Pulmonary embolism
  • On CT angiography:
    • Intra-luminal filling defect
  • On MRI:
    • Narrowing of involved vessel
    • No contrast seen distal to obstruction
    • Polo-mint sign (partial filling defect surrounded by contrast)
✔ (Low grade) ✔ (In case of massive PE) - - - -
Congestive heart failure
  • Goldberg's criteria may aid in diagnosis of left ventricular dysfunction: (High specificity)
    • SV1 or SV2 + RV5 or RV6 ≥3.5 mV
    • Total QRS amplitude in each of the limb leads ≤0.8 mV
    • R/S ratio <1 in lead V4
- - - - - -
  • ST elevation
  • PR depression
  • Large collection of fluid inside the pericardial sac (pericardial effusion)
  • Calcification of pericardial sac
✔ (Low grade) ✔ (Relieved by sitting up and leaning forward) - - - - -
  • May be clinically classified into:
    • Acute (< 6 weeks)
    • Sub-acute (6 weeks - 6 months)
    • Chronic (> 6 months)
Pneumonia - - - -

Homogeneous, circumferential vessel wall swelling

Chronic obstructive pulmonary disease (COPD)
  • On CT scan:
  • On MRI:
    • Increased diameter of pulmonary arteries
    • Peripheral pulmonary vasculature attentuation
    • Loss of retrosternal airspace due to right ventricular enlargement
    • Hyperpolarized Helium MRI may show progressively poor ventilation and destruction of lung
- - - - - -


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