What labs are recommended for diagnosing gout?

Recommended Laboratory Tests for Diagnosing 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

Primary Diagnostic Tests

• Synovial fluid analysis: Examination for MSU crystals should be performed for a definite diagnosis of gout, with sensitivity of 84% and specificity approaching 100% 1, 2

  • MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy 2
  • Even during intercritical periods (between attacks), synovial fluid analysis can detect MSU crystals in approximately 70% of patients with known gout 1, 3
  • Gram stain and culture should still be performed even if MSU crystals are identified, as gout and sepsis may coexist 2, 4

• Serum uric acid (SUA) measurement: Essential but insufficient alone for diagnosis 1, 2

  • A cut-off level of 6 mg/dL has moderate sensitivity (0.67) and specificity (0.78) 3
  • The diagnosis of gout should not be made on the presence of hyperuricemia alone 1

Additional Laboratory Tests

• Renal function tests: Should be measured in all patients with gout and/or hyperuricemia 1

  • Chronic kidney disease is a risk factor for gout and may influence treatment decisions 1, 2

• Cardiovascular risk factor assessment: Recommended for all gout patients 1

  • Including lipid profile and glucose/HbA1c 1, 2

• Renal uric acid excretion: Should be determined in selected patients 3

  • Particularly those with family history of young-onset gout, onset under age 25, or history of renal calculi 3
  • Can be measured using a 24-hour urine collection or early morning spot urine sample for uric acid to creatinine ratio 1

Imaging Studies

• Ultrasound: More helpful than plain radiographs for diagnosis 1, 3

  • Can detect tophi not evident on clinical examination 1
  • The "double contour sign" at cartilage surfaces is highly specific for urate deposits 1, 2
  • Sensitivity ranges from 37% to 100% and specificity from 68% to 97% 2

• Dual-Energy CT (DECT): High sensitivity (85-100%) and specificity (83-92%) for detecting MSU crystal deposition 2

• Plain radiographs: Limited value for diagnosing acute gout but may show typical features in chronic gout 1, 2

  • Useful for differential diagnosis and may show asymmetrical swelling and subcortical cysts in chronic cases 1

Clinical Diagnosis When Crystal Analysis Not Available

If synovial fluid analysis is not possible, a clinical diagnosis can be supported by:

• Monoarticular involvement of a foot or ankle joint 3, 2
• Previous similar acute arthritis episodes 3, 2
• Rapid onset of severe pain and swelling reaching maximum intensity within 24 hours 3, 2
• Presence of erythema 3, 2
• Male gender 3, 2
• Associated cardiovascular diseases 3, 2
• Hyperuricemia 3, 2

Common Pitfalls and Caveats

• Inadequate aspiration technique may yield false-negative results 4
• Delayed analysis of synovial fluid may lead to crystal dissolution 4
• Relying solely on serum uric acid levels for diagnosis is insufficient, as some patients with acute gout may have normal levels during an attack 1, 2
• Failing to assess for coexistent septic arthritis, as gout and joint infection can occur simultaneously 1, 2

By following this diagnostic approach, clinicians can accurately diagnose gout and initiate appropriate treatment to reduce morbidity and improve quality of life for patients.