What tests should be obtained to diagnose gout?

Diagnostic Tests for 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

Diagnostic Algorithm for Gout

First-Line Diagnostic Test

• Synovial fluid aspiration and examination for MSU crystals is strongly recommended as the first diagnostic step for any patient with suspected gout 2
• MSU crystal identification has extremely high specificity (100%) and good sensitivity (84%) for diagnosing gout 2
• Even during intercritical periods (between attacks), synovial fluid analysis can detect MSU crystals in approximately 70% of patients with known gout 2
• Gram stain and culture of synovial fluid should still be performed even if MSU crystals are identified, as gout and sepsis may coexist 2

When Synovial Fluid Analysis Is Not Feasible

If crystal identification is not possible, a clinical diagnosis can be supported by:

• Monoarticular involvement of a foot (especially the first metatarsophalangeal joint) or ankle joint 2
• Previous similar acute arthritis episodes 2
• Rapid onset of severe pain and swelling reaching maximum intensity within 24 hours 2
• Presence of erythema 2
• Male gender 2
• Associated cardiovascular diseases 2
• Hyperuricemia 2

Laboratory Tests

• Serum uric acid (SUA) measurement is essential but insufficient alone for diagnosis 1

• Important caveats regarding SUA:

  • Many people with hyperuricemia never develop gout 2
  • During acute attacks, serum levels may be normal 2
  • A cut-off level of 6 mg/dL (360 μmol/L) has moderate sensitivity (0.67) and specificity (0.78) 2
  • Gender-specific cut-offs (>7 mg/dL for men, >6 mg/dL for women) improve diagnostic accuracy 2

• Renal uric acid excretion should be determined in selected patients, particularly those with:

  • Family history of young-onset gout 2
  • Onset of gout under age 25 2
  • History of renal calculi 2

Imaging Studies

• When clinical diagnosis is uncertain and crystal identification is not possible, imaging should be used to search for MSU crystal deposition 2

• Plain radiographs:

  • Limited value for diagnosis of early or acute gout 2
  • May show typical features in chronic gout 2
  • Useful for differential diagnosis 2

• Ultrasound:

  • More helpful than plain radiographs for diagnosis 2
  • Can detect tophi not evident on clinical examination 2
  • Can identify double contour sign at cartilage surfaces (highly specific for urate deposits) 2

Additional Assessments

• Risk factors for chronic hyperuricemia should be evaluated in all gout patients:

  • Chronic kidney disease 2
  • Overweight/obesity 2
  • Medications (diuretics, low-dose aspirin, cyclosporine, tacrolimus) 2
  • Alcohol consumption (particularly beer and spirits) 2
  • Dietary factors (non-diet sodas, meat, shellfish) 2

• Systematic assessment for associated comorbidities:

  • Obesity 2
  • Renal impairment 2
  • Hypertension 2
  • Ischemic heart disease 2
  • Heart failure 2
  • Diabetes 2
  • Dyslipidemia 2

Common Pitfalls to Avoid

• Relying solely on serum uric acid for diagnosis - SUA may be normal during acute attacks 1
• Failing to perform synovial fluid analysis when possible - this is the gold standard test 2
• Not considering gout in the differential diagnosis of acute monoarthritis 2
• Overlooking the need to assess for septic arthritis, which can coexist with gout 2
• Not evaluating cardiovascular risk factors in patients with gout 1
• Failing to monitor SUA levels regularly during treatment 1