What lab workup is recommended for a patient suspected of having gout?

Lab Workup for Gout

The definitive diagnosis of gout requires synovial fluid aspiration with identification of monosodium urate (MSU) crystals under polarized light microscopy, which remains the gold standard with sensitivity and specificity approaching 100%. 1, 2

Essential Laboratory Tests

Synovial Fluid Analysis (Gold Standard)

• Joint aspiration with synovial fluid analysis should be performed in all patients with undiagnosed inflammatory arthritis, particularly when the diagnosis is uncertain, septic arthritis cannot be excluded, or this is the first suspected gout attack 1, 2
• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy with 84% sensitivity and 100% specificity 1, 3
• Gram stain and culture must be performed even when MSU crystals are identified, as gout and septic arthritis can coexist 1, 3
• During intercritical periods (between attacks), synovial fluid analysis can still detect MSU crystals in approximately 70% of patients with known gout 1, 3

Serum Uric Acid Level

• Serum uric acid should be measured, but hyperuricemia alone cannot diagnose gout and normal levels do not exclude it 1, 2
• Hyperuricemia has only 53-61% specificity for gout diagnosis, meaning many hyperuricemic patients never develop gout 1
• Only 22% of asymptomatic patients with serum uric acid levels above 9 mg/dL develop gout over 5 years 1
• Conversely, approximately 10% of patients with acute gout have normal serum uric acid levels during an attack 1
• The absence of hyperuricemia has a markedly low likelihood ratio, making gout very unlikely but not completely excluded 1, 2
• Gender-specific cut-offs (>7 mg/dL for males, >6 mg/dL for females) improve diagnostic accuracy with sensitivity 57% and specificity 92% 1, 3

Renal Function Assessment

• Serum creatinine and estimated glomerular filtration rate (eGFR) should be measured to assess for chronic kidney disease, which has a relative risk of 4.95 for gout 2, 3
• Renal function testing guides medication selection and dosing for both acute and chronic management 2

24-Hour Urine Uric Acid Excretion (Selected Patients)

• Renal uric acid excretion should be determined in patients with:
  • Family history of young-onset gout 1, 3
  • Onset of gout under age 25 1, 3
  • History of renal calculi 1, 3
• This test helps distinguish underexcretion from overproduction of uric acid and guides urate-lowering therapy selection 3

Inflammatory Markers

• Complete blood count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) should be obtained to assess inflammation severity and help rule out infection 2

Risk Factor and Comorbidity Assessment

All patients with suspected or confirmed gout should be systematically screened for associated comorbidities and risk factors: 1, 2, 3

• Cardiovascular risk factors: hypertension (relative risk 3.93), obesity (relative risk 3.81), diabetes, dyslipidemia 2, 3
• Medication review: diuretics (relative risk 1.72), low-dose aspirin, cyclosporine, tacrolimus 2, 3
• Dietary assessment: alcohol consumption (especially beer), meat and shellfish intake, high-fructose corn syrup consumption 2, 3, 4
• Renal impairment: chronic kidney disease significantly increases gout risk 2, 3

When Clinical Diagnosis Without Aspiration Is Acceptable

For typical recurrent podagra (first metatarsophalangeal joint involvement) with hyperuricemia and characteristic features, clinical diagnosis has 95% strength of recommendation, though it is not definitive without crystal confirmation 1

Clinical features with highest diagnostic value include: 1, 3

• Podagra (sensitivity 96%, specificity 97%, likelihood ratio 30.64)
• Definite tophus (sensitivity 30%, specificity 99%, likelihood ratio 39.95)
• Maximum pain within 24 hours of onset
• Erythema over the affected joint (sensitivity 92%, specificity 62%)
• Male gender
• History of previous similar attacks

Common Pitfalls to Avoid

• Do not diagnose gout based on hyperuricemia alone—up to 80% of patients with hyperuricemia never develop gout 1, 2
• Do not exclude gout based on normal serum uric acid during an acute attack—approximately 10% of gout patients have normal levels during flares 1
• Never skip Gram stain and culture when performing joint aspiration—crystal arthritis and septic arthritis can coexist 1, 3
• Joint aspiration is technically difficult in smaller joints and rarely performed in primary care, but should still be attempted when diagnosis is uncertain or infection cannot be excluded 2
• Amorphous urate crystals in urinalysis are not diagnostic of gout and represent a benign finding of concentrated, acidic urine 5