What uric acid level is significant for diagnosing gout?

Serum Uric Acid Has Limited Diagnostic Value for Gout

Serum uric acid levels do not confirm or exclude the diagnosis of gout, and no specific uric acid threshold should be used to diagnose gout. 1 The gold standard for definitive diagnosis remains demonstration of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates under polarized light microscopy. 1, 2, 3

Why Uric Acid Levels Are Unreliable for Diagnosis

Normal Levels During Acute Attacks Are Common

• 14-32% of patients with acute gout have serum uric acid ≤6-8 mg/dL during the attack itself. 4
• In one study, 63.3% of patients with acute gout attacks had normal serum uric acid levels at presentation. 5
• Uric acid behaves as a negative acute phase reactant, temporarily decreasing during episodes of acute inflammation and stress. 1, 2
• The mechanism involves increased renal excretion of uric acid during acute inflammatory episodes. 2

Hyperuricemia Alone Is Not Diagnostic

• Many people with hyperuricemia never develop gout—only 22% of asymptomatic patients with serum uric acid >9 mg/dL develop gout over 5 years. 2
• Approximately 15-25% of people with asymptomatic hyperuricemia have asymptomatic MSU crystal deposition without clinical gout. 1, 2
• The specificity of hyperuricemia for diagnosing gout is relatively low. 2

The Correct Diagnostic Approach

Crystal Identification Is the Gold Standard

• Synovial fluid aspiration with MSU crystal identification under polarized light microscopy permits definitive diagnosis (strength of recommendation: 96/100). 3
• Crystal identification from asymptomatic joints during intercritical periods also allows definite diagnosis (strength of recommendation: 84/100). 3
• A routine search for MSU crystals is recommended in all synovial fluid samples from undiagnosed inflamed joints (strength of recommendation: 90/100). 3

When to Measure Serum Uric Acid

• Serum uric acid should be measured at distance from an acute gout flare, not during the attack. 1, 2
• The measurement is useful for monitoring treatment targets (goal <6 mg/dL), not for diagnosis. 3, 6, 7
• The theoretical saturation point for MSU crystal formation is approximately 6.8 mg/dL. 2

Clinical Features Supporting Gout Diagnosis

• Rapid development of severe pain reaching maximum within 6-12 hours. 2
• Podagra (first metatarsophalangeal joint involvement) with overlying erythema. 2
• Clinical algorithms have sensitivities of 85-92% and specificities of 78-89% when MSU crystals cannot be identified. 1

Critical Pitfalls to Avoid

Do Not Exclude Gout Based on Normal Uric Acid

• The absence of hyperuricemia after an acute arthritis episode has a markedly low negative likelihood ratio. 3
• The diagnosis should not be excluded based on normal serum uric acid if clinical manifestations are suggestive. 1, 2, 3

Do Not Rely on Uric Acid Alone for Diagnosis

• Serum uric acid has limited diagnostic value, especially during acute flares. 1, 2
• When clinical diagnosis is uncertain and crystal identification is not possible, imaging (ultrasound, DECT) should be used to search for MSU crystal deposition. 1

Remember Coexisting Conditions

• Gout and septic arthritis may coexist, so Gram stain and culture should still be performed even if MSU crystals are identified when infection is suspected (strength of recommendation: 93/100). 3

Imaging When Crystal Identification Is Not Feasible

Ultrasound Features

• Double contour sign: sensitivity 50-83%, specificity 76-92%. 1
• Tophus detection: sensitivity 33-65%, specificity 80-95%. 1
• Performs better in late disease (≥2 years) than early disease (<2 years). 1

Dual-Energy CT (DECT)

• Pooled sensitivity: 84-87%, specificity: 84-93%. 1
• Useful for detecting MSU crystal deposition when synovial fluid aspiration is not feasible. 1