What lab tests indicate an acute gout attack?

Laboratory Tests for Diagnosing Acute Gout Attack

The definitive diagnosis of an acute gout attack requires demonstration of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates. 1

Gold Standard Diagnostic Test

• Synovial fluid analysis: The American College of Physicians recommends synovial fluid analysis when clinical judgment indicates that diagnostic testing is necessary in patients with possible acute gout 1. This involves:
  • Identification of needle-shaped, negatively birefringent MSU crystals under polarized light microscopy
  • During acute attacks, synovial fluid analysis has a sensitivity of 84% and specificity of 100% (likelihood ratio 566.60) 1
  • Even during intercritical periods, MSU crystal identification has a sensitivity of 70% and specificity of 95% 1

Serum Uric Acid Testing

• Serum uric acid (SUA) levels are not reliable for diagnosing acute gout attacks 1, 2
• Important limitations of SUA testing:
  • SUA may be normal during acute gout attacks in up to 63.3% of patients 3
  • While hyperuricemia is the most important risk factor for gout, it does not confirm or exclude gout 1
  • Many people with hyperuricemia never develop gout 1
  • The mean SUA level in acute gout is approximately 8.4 mg/dL, but with significant variation 4

Inflammatory Markers

• Elevated inflammatory markers are commonly seen during acute gout attacks but are not specific:
  • Erythrocyte sedimentation rate (ESR) is elevated in approximately 76.6% of patients 3
  • C-reactive protein (CRP) is elevated in approximately 30% of patients 3
  • These markers help assess the severity of inflammation but cannot differentiate gout from other inflammatory arthritides

Urinary Uric Acid Excretion

• Urinary uric acid excretion may be increased during acute attacks in approximately 55% of patients 3
• Testing is recommended in selected patients, particularly those with:
  • Family history of young-onset gout
  • Onset of gout under age 25
  • History of renal calculi 1

Diagnostic Algorithm

1. First-line approach: Joint aspiration with polarized light microscopy to identify MSU crystals (definitive diagnosis) 2

2. If joint aspiration is not feasible, diagnosis relies on clinical features:

   • Rapid development of severe pain, swelling, and tenderness reaching maximum within 6-12 hours, especially with overlying erythema 1
   • Podagra (first MTP joint involvement) - likelihood ratio 30.64 2
   • Presence of tophi - likelihood ratio 39.95 for definite tophus 2
   • Hyperuricemia (though not diagnostic by itself) - likelihood ratio 9.74 2
   • History of similar previous attacks with complete resolution

3. Supportive imaging: If clinical diagnosis is uncertain and joint aspiration is not possible:

   • Radiographs are not useful in confirming early or acute gout 1
   • Only grade IV radiographic findings (intraosseous calcific deposits, subperiosteal bone apposition) have meaningful diagnostic value 2

Common Pitfalls in Diagnosis

• Relying solely on serum uric acid levels - normal levels do not exclude acute gout 1, 3
• Failure to perform synovial fluid analysis when feasible - missing the definitive diagnosis 2
• Overreliance on non-specific inflammatory markers - these cannot differentiate gout from other inflammatory conditions 3
• Misinterpreting radiographic findings - early radiographic findings have poor specificity 2
• Not considering gout in patients without hyperuricemia - approximately 63% of acute gout attacks can occur with normal SUA levels 3

By following this diagnostic approach with emphasis on crystal identification whenever possible, clinicians can accurately diagnose acute gout attacks and initiate appropriate treatment to reduce morbidity and improve quality of life.