How to differentiate a bunion with bursitis from a gout flare?

Distinguishing Bunion with Bursitis from Gout Flare

The definitive way to distinguish bunion with bursitis from a gout flare is through synovial fluid aspiration and crystal analysis—if monosodium urate (MSU) crystals are identified under polarizing microscopy, the diagnosis is gout; if no crystals are present, it is bursitis. 1

Gold Standard Diagnostic Approach

Synovial fluid aspiration with crystal analysis should be performed in any patient with undiagnosed inflammatory arthritis, including suspected bunion bursitis versus gout. 1 This is critical because:

• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy with sensitivity of 63-78% and specificity of 93-100% 2
• Gout and bacterial infection can coexist in the same bursa, making clinical diagnosis alone unreliable 3
• Septic bursitis must be ruled out, as it requires different treatment and can be life-threatening if missed 4, 3

Clinical Features That Favor Gout

When synovial fluid analysis is not immediately available, certain clinical features strongly suggest gout over simple bursitis:

• Rapid onset of severe pain reaching maximum intensity within 6-12 hours (not days) 2
• Podagra (first metatarsophalangeal joint involvement) has the highest predictive value 1, 2
• Visible tophi on examination (odds ratio 7.29 for gout diagnosis) 2
• Overlying erythema with severe tenderness disproportionate to examination findings 2
• Resolution within 2 weeks even without treatment 2
• Hyperuricemia (though this alone cannot diagnose gout, as only 22% of patients with uric acid >9 mg/dL develop gout within 5 years) 1

Clinical Features That Favor Bursitis

• Chronic microtrauma history (e.g., prolonged kneeling, repetitive pressure on the bunion area) 4
• Gradual onset over days rather than hours 4
• Localized swelling without the intense erythema typical of crystal inflammation 4
• Absence of systemic risk factors for gout (male sex, obesity, hypertension, diuretic use, diet rich in meat/seafood) 2

Advanced Imaging When Aspiration Is Not Feasible

If joint aspiration is technically difficult or unavailable, imaging can provide diagnostic clarity:

• Ultrasound can identify the double contour sign (urate deposition on cartilage) or tophi with 74% sensitivity and 88% specificity for gout 1
• Dual-energy CT (DECT) shows 85-100% sensitivity and 83-92% specificity for detecting MSU crystal deposits 1
• When characteristic features of MSU crystal deposition are identified on ultrasound or DECT, synovial fluid analysis is not needed to confirm gout 1
• Ultrasound guidance should be used for aspiration when anatomical landmarks make the procedure challenging 1

Critical Pitfall: Coexistent Infection

A major diagnostic pitfall is assuming all inflamed bursae in gout patients are purely crystal-induced. 3 In patients with risk factors for infection (elderly, immunosuppressed, diabetes, chronic kidney disease), aspiration is mandatory because:

• Gout crystals and bacterial infection can coexist in the same bursa 3
• Treating presumed gout with corticosteroids when infection is present can be catastrophic 3
• Bursal fluid should undergo Gram stain, culture, glucose measurement, cell count, and crystal analysis 4

Clinical Algorithm When Aspiration Is Unavailable

If aspiration cannot be performed immediately, clinical algorithms incorporating multiple features have >80% sensitivity and specificity compared to crystal analysis 1:

1. More than one attack of acute arthritis in the patient's history
2. Maximum inflammation developing within 1 day
3. Redness observed over the joint
4. First metatarsophalangeal joint involvement
5. Proven or suspected tophi
6. Hyperuricemia present

These algorithms perform better in patients with disease duration >2 years than in those with recent-onset symptoms ≤2 years 1, 2

When to Treat Empirically

Hyperuricemia alone should never be used to diagnose gout or guide treatment decisions. 1 However, in a patient with classic podagra, rapid onset, prior documented gout attacks, and no signs of infection, empiric treatment for gout may be reasonable while arranging definitive testing 1. Conversely, if there is any suspicion of septic bursitis (fever, severe systemic symptoms, immunocompromise), aspiration is mandatory before initiating any anti-inflammatory therapy 4, 3.