Best Test to Diagnose Gout
Synovial fluid aspiration with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard diagnostic test for gout, demonstrating exceptional specificity (100%) and high sensitivity (84%) with a likelihood ratio of 566.60. 1
When to Perform Synovial Fluid Analysis
You should perform joint aspiration when:
- The diagnosis is clinically uncertain or ambiguous 1
- There is any suspicion of septic arthritis (gout and sepsis can coexist in the same joint) 1
- The patient has not had a prior crystal-proven diagnosis 1
- An experienced clinician is available who can aspirate the joint without substantial patient discomfort 1
- A polarizing microscope and trained operator are accessible 1, 2
You can rely on clinical judgment alone when:
- The presentation is classic (podagra with appropriate risk factors like male sex, hyperuricemia, no overlying skin wound) 1
- No significant probability of infection exists 1
- Joint aspiration would cause substantial patient discomfort or is technically not feasible 1
Critical Technical Requirements
The test requires specific expertise and equipment that significantly impacts accuracy 1:
- Polarizing microscope to identify needle-shaped crystals with strong negative birefringence 3, 4
- Trained operator experienced in crystal identification (inter-observer variability exists) 1
- Proper specimen handling to preserve crystal integrity 2
Essential Pitfall to Avoid
Always perform Gram stain and synovial fluid culture even when MSU crystals are identified if septic arthritis is in the differential diagnosis. 1, 4 In reported case series, 73-76% of patients with coexistent gout and septic arthritis had positive cultures, and some required amputation or died from untreated infection 1
When Synovial Fluid Analysis Cannot Be Performed
If joint aspiration is not feasible, validated clinical prediction algorithms can achieve sensitivities and specificities greater than 80% 1, 3:
- The 2015 ACR/EULAR classification criteria (score ≥8 points) demonstrate 92% sensitivity and 89% specificity with imaging 3
- Key high-value clinical features include presence of tophi (odds ratio 7.29) and resolution of pain within 2 weeks (odds ratio 3.58) 3
Alternative Imaging Modalities
Dual-Energy CT (DECT) is the most accurate non-invasive test with sensitivity 85-100% and specificity 83-92% 2, 3, but current evidence is insufficient for the American College of Physicians to recommend it for routine acute gout diagnosis 1
Ultrasonography shows variable performance (sensitivity 37-100%, specificity 68-97%) depending on the specific signs assessed 2, 3, with the "double contour" sign being most specific (pooled specificity 88%) 3
Intercritical Period Diagnosis
MSU crystals can be identified in asymptomatic joints during pain-free intervals, with approximately 70% positive yield in previously affected joints 1, allowing definitive diagnosis even between acute attacks 1
What NOT to Rely On
Serum uric acid levels do not confirm or exclude gout 1:
- Many people with hyperuricemia never develop gout 1
- During acute attacks, serum uric acid may be normal 1
- Serum uric acid alone has insufficient evidence for diagnostic utility 1
Plain radiographs are not useful for confirming early or acute gout 1, though they may show typical features in chronic disease and help with differential diagnosis 1