How do you differentiate bursitis from gout (gouty arthritis) in terms of diagnosis and treatment?

Differentiating Bursitis from Gout: Diagnostic Approach

Synovial fluid aspiration with polarized light microscopy for monosodium urate (MSU) crystals is the definitive test to distinguish gout from bursitis, and should be performed in any patient with undiagnosed inflammatory arthritis or suspected septic bursitis. 1

Gold Standard Diagnostic Test

• Bursal or joint fluid aspiration with crystal analysis under polarized light microscopy is the reference standard, with MSU crystals appearing as needle-shaped, negatively birefringent crystals 1, 2
• Sensitivity and specificity approach 100% when performed by experienced operators 2
• Gram stain and culture must be performed even when MSU crystals are identified, as gout and septic bursitis can coexist 3, 2, 4

Key Clinical Features That Distinguish Gout from Bursitis

Gout-Specific Features:

• Maximum pain intensity reached within 6-12 hours (often within 24 hours) strongly suggests gout over traumatic bursitis 2
• True joint involvement with joint line tenderness and restricted range of motion indicates gouty arthritis rather than isolated bursitis 2
• Overlying erythema combined with rapid-onset severe pain is highly suggestive of gout 2
• Recurrent pattern with more than one previous attack of acute arthritis strongly favors gout 2
• First metatarsophalangeal joint involvement (podagra) has the highest diagnostic value (likelihood ratio = 30.64) 1
• Presence of tophi has likelihood ratio of 39.95 for gout 1

Bursitis-Specific Features:

• Chronic microtrauma history (e.g., repetitive kneeling for prepatellar bursitis) is the most common cause of non-gouty bursitis 5
• Acute trauma or hemorrhage suggests traumatic bursitis 5
• Pain develops more gradually over days rather than hours 5

Bursal Fluid Analysis Characteristics

When aspiration is performed, specific fluid characteristics help differentiate:

• Gouty bursitis: Yellow or pink fluid in most cases, chalky white fluid occasionally, or small amount of bloody fluid; leucocyte count averages 2.9 × 10⁹/L (significantly lower than articular gout which averages 25.5 × 10⁹/L) 6
• Septic bursitis: Purulent fluid with positive Gram stain/culture; Staphylococcus aureus is the most common organism 5, 4
• Traumatic bursitis: Bloody or serosanguinous fluid without crystals or organisms 5

Laboratory Testing Algorithm

Step 1: Attempt Bursal/Joint Aspiration

• Mandatory when septic bursitis cannot be clinically excluded 2, 5
• Required in atypical presentations or when diagnosis is uncertain 1, 2
• Send fluid for: crystal analysis under polarized microscopy, Gram stain, culture, cell count with differential, and glucose measurement 3

Step 2: Serum Uric Acid Measurement

• Serum uric acid alone is insufficient for diagnosis because many hyperuricemic patients never develop gout, and 30-40% of patients with acute gout have normal serum uric acid during the attack 1, 3, 2
• Hyperuricemia (defined by local population normal range) has likelihood ratio of 9.74 when present, but normal levels do not exclude gout 1, 7
• Useful for ongoing management once diagnosis is established 3

Step 3: Imaging When Aspiration Not Feasible

• Ultrasound: "Double contour sign" at cartilage surfaces has 74% sensitivity and 88% specificity for MSU crystal deposition 1, 2
• Dual-energy CT (DECT): 85-100% sensitivity and 83-92% specificity for detecting MSU crystals, but less sensitive in early disease or shorter symptom duration 2
• Plain radiography: Limited value for acute diagnosis but useful for chronic changes (asymmetrical swelling, subcortical cysts without erosion) and ruling out other conditions 1, 2

Risk Factors Supporting Gout Diagnosis

Presence of multiple risk factors increases likelihood of gout:

• Chronic kidney disease (relative risk 4.95) 2
• Hypertension (relative risk 3.93) 2
• Obesity (relative risk 3.81) 2
• Diuretic use (relative risk 1.72) 1, 2
• Male sex, alcohol consumption (especially beer), diet high in meat/seafood 1, 2

When Clinical Diagnosis Alone Is Acceptable

For typical recurrent podagra with hyperuricemia and no fever or systemic signs suggesting infection, clinical diagnosis is reasonably accurate (>80% sensitivity and specificity when multiple characteristic features present) 1, 2

Clinical algorithms incorporating rapid onset (<24 hours), redness over affected joint, painful/swollen first MTP joint, and hyperuricemia achieve >80% sensitivity and specificity 2

Critical Pitfalls to Avoid

• Never rely on serum uric acid alone—it may be normal during acute attacks and hyperuricemia alone does not confirm gout 1, 3
• Never assume gout based on hyperuricemia in a patient with bursitis—aspiration is required for definitive diagnosis 1
• Always consider septic bursitis in the differential—the relatively low specificity of clinical algorithms creates risk for missed septic arthritis, which requires immediate antibiotic therapy 2, 5
• Repeated aspiration may be necessary in elusive cases, as initial aspirations can be non-diagnostic 7
• Elderly patients, those with severe comorbidity, or immunodeficiency require aspiration even when gout seems likely, due to increased risk of concurrent infection 4