What is the diagnostic approach for gout?

Diagnosis of Gout

The definitive diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirate using polarized light microscopy, which is the gold standard diagnostic test. 1, 2

Three-Step Diagnostic Approach

Step 1: Synovial Fluid Analysis (Gold Standard)

• Joint aspiration and synovial fluid analysis for MSU crystals is the reference standard for diagnosing gout 1, 3
• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy 2
• Synovial fluid analysis has sensitivity and specificity approaching 100% when performed by experienced operators 1, 3
• Even when MSU crystals are identified, Gram stain and culture should still be performed as gout and septic arthritis may coexist 2

Step 2: Clinical Diagnosis (When Synovial Fluid Analysis Not Possible)

When crystal identification is not possible, clinical features highly suggestive of gout include:

• Monoarticular arthritis affecting the first metatarsophalangeal joint (podagra) 1
• Rapid onset of severe pain reaching maximum intensity within 24 hours 1
• Erythema over the affected joint 1
• Male gender 1
• History of previous similar attacks 1
• Cardiovascular comorbidities 1
• Hyperuricemia (though not diagnostic on its own) 1, 2

Several clinical prediction tools have demonstrated sensitivities and specificities greater than 80% compared to synovial fluid analysis 1, 3

Step 3: Imaging (When Diagnosis Remains Uncertain)

If clinical diagnosis is uncertain and crystal identification not possible, imaging should be used to search for MSU crystal deposition:

• Ultrasound:

  • Sensitivity ranges from 37% to 100% and specificity from 68% to 97% 1
  • The "double contour sign" (hyperechoic irregular enhancement over hyaline cartilage) has pooled sensitivity of 74% and specificity of 88% 1
  • Particularly helpful in detecting tophi not evident on clinical examination 1

• Dual-Energy CT (DECT):

  • Sensitivity ranges from 85% to 100% and specificity from 83% to 92% 1
  • Less sensitive in patients with recent-onset acute gout 1
  • False positives may occur in patients with advanced osteoarthritis 1

• Plain Radiographs:

  • Limited value for diagnosing acute gout flares 1
  • Useful for identifying chronic changes and ruling out other conditions 1

Important Considerations

Serum Uric Acid Testing

• Serum uric acid level alone is insufficient for diagnosis 1, 2
• Many patients with hyperuricemia never develop gout 2
• Some patients with acute gout may have normal serum uric acid levels during an attack 2
• Serum uric acid measurement is essential for ongoing management and monitoring treatment efficacy 2

Risk Factors Assessment

Risk factors that should be evaluated include:

• Chronic kidney disease 1
• Overweight/obesity 1
• Medications (diuretics, low-dose aspirin, cyclosporine, tacrolimus) 1, 2
• Alcohol consumption (particularly beer and spirits) 1
• Diet high in meat and shellfish 1
• Non-diet sodas and high-fructose corn syrup consumption 1, 2

Comorbidity Assessment

Systematic assessment for associated comorbidities is recommended:

• Obesity 1
• Renal impairment 1
• Hypertension 1
• Ischemic heart disease 1
• Heart failure 1
• Diabetes 1
• Dyslipidemia 1

Common Pitfalls to Avoid

• Relying solely on serum uric acid levels for diagnosis 1, 2
• Failing to perform synovial fluid analysis in atypical presentations 2
• Not considering septic arthritis as a differential diagnosis or potential coexisting condition 2
• Overlooking the fact that DECT and ultrasound have lower sensitivity in early/recent-onset gout 1
• Not accounting for medications that can affect serum urate levels when interpreting results 2