Sandbox:SA diagnosis

Diagnosis

Patients with history of chronic disease with concurrent septic arthritis can be misdiagnosed as acute flareup of underlying chronic disease which often delays the treatment for septic arthritis. So, patients with acute flare of one or two new inflamed joints with underlying chronic joint diseases or with another connective tissue disease, it should be assumed that the joint is septic until proven otherwise, should always rule out concurrent septic arthritis with appropriate diagnostic studies.

In patients with acute effusion of unknown etiology, might have concurrent crystal-induced arthritis and septic arthritis. So, the synovial fluid should always be cultured and examined for crystals in the evaluation of an acute effusion.

History and Symptoms

Abrupt onset of a single hot, swollen, and painful joint indicate non gonococcal arthritis. It can involve any joint, but most commonly knee is the site of infection in 50% of cases of adults and elderly patients. Hip infection is the most common site in children.

Disseminated gonococcal infection(DGI) often present initially with migratory polyarthralgias, tenosynovitis, dermatitis, and fever and less commonly, <50% of patients with DGI will present with purulent joint effusion, most often of the knee or wrist.. Often present with inflamed and tender tendons of the wrist, ankles, and small joints.

Appearance of the Patient

Patient with septic arthritis usually appears toxic and with joint pain that involved

Vital Signs

• Low grade fever. Chills and spiking fever are very rare.
• Hyperthermia over the joint involved
• Tachycardia
• Tachypnea

Skin

• Warm over the joint

• Erythema over the around the joint that involved
• Disseminated gonococcal infection often present with skin lesions, typically multiple, painless macules and papules, most often found on the arms or legs or on the trunk.

Extremities

Most commonly involves knee > hip > shoulder > ankle. Other joints such as sacroiliac joint (~10%), sternoclacicular or costoclavicular joints may be involved in patient with history of intravenous drug abuse (IVDA), penetrating trauma, animal or human bites and local steroid injections.

• Swelling of the joint that involved
• Decreased range of motion
• Patient hold the hip in flexed and externally rotated position if SA involving hip.

Serum markers

Serum markers such as peripheral white cell count, erythrocyte sedimentation ratio, C-reactive protein are useful to determine infectious or inflammatory response and also useful to monitor therapeutic response. Absence of raise in these parameters may not be good correlate for the diagnosis of septic arthritis, as these tests are neither sensitive nor specific.

• Peripheral WBC count: Peripheral WBC count increases in septic arthritis, especially in children where as in adults it varies with severity. It is not sensitive or specific to diagnose septic arthritis.
• Erythrocyte sedimentation ratio: Patients with septic arthritis may have ESR > 30 mm/hr, but normal ESR may not ruleout septic arthritis.
• C- reactive protein: Elevated CRP of > 100 mg/L increased the likelihood of septic arthritis slightly.

Synovial Fluid Analysis

Diagnosis of septic arthritis mainly depends on arthrocentesis and isolation of the pathogen from aspirated joint fluid. Clinical suspicion of joint sepsis should prompt for immediate synovial fluid aspiration. Septic arthritis should not be excluded even though the patient have low fever and normal WBC. The definitive diagnosis of septic arthritis requires identification of bacteria in the synovial fluid by Gram’s stain or by culture. If synovial fluid cannot be obtained with closed needle aspiration, the joint should be aspirated again with imaging guidance such as ultrasound guidance, computed tomography or fluoroscopic guidance. Synovial fluid analysis include:

• Synovial WBC count with differential
• Crystal analysis
• Gram stain
• Culture and sensitivity

Normal synovial fluid appears as clear, transparent, thick in viscosity with WBC count less than 200 mm3 and < 25% of PMN, where as in septic arthritis and other arthritis synovial fluid analysis will be as follows:

Type of Arthritis	Color	Transparency	Viscosity	WBC count (per mm3)	PMN cellcount (%)	Gram stain	Gram Culture	polymerase chain reaction (PCR) test	Crystals
Normal	Clear	Transparent	High/thick	< 200	< 25	Negative	Negative	Negative	Negative
Gonococcal arthritis	Yellow	Cloudy-opaque	Low	34,000 to 68,000	> 75	Variable (< 50 percent)	Positive (25 to 70 percent)	Positive (> 75 percent)	Negative
Non-gonococcal arthritis	Yellowish-green	Opaque	Very low	> 50,000 (> 100,000 is more specific)	> 75	Positive (60 to 80 percent)	Positive (> 90 percent)	--	Negative
Inflammatory: crystalline arthritis (e.g.Gout, Pseudo gout)	Yellow	Cloudy	Low/thin	2,000 to 100,000	> 50	Negative	Negative	Negative	Positive
Inflammatory: non-crystalline arthritis (e.g. Rheumatoid arthritis, reactive arthritis)	Yellow	Cloudy	Low/thin	2,000 to 100,000	> 50	Negative	Negative	Negative	Negative
Noninflammatory arthritis (e.g. Osteoarthritis)	Straw	Translucent	High/thick	200 to 2,000	< 25	Negative	Negative	Negative	Negative
Lyme arthritis	Yellow	Cloudy	Low	3,000 to 100,000 (mean: 25,000)	> 50	Negative	Negative	Positive (85 percent)	Negative

• Synovial fluid glucose level < 40 mg/dl and increased lactate level may represent septic arthritis, but these parameters are very less sensitive.
• Presence of crystals may not exclude septic arthritis, as the coexistant infection might be possible along with crystalline disease.