What laboratory tests are recommended for a patient suspected of having gout?

Laboratory Tests for Suspected Gout

The definitive diagnostic test is synovial fluid aspiration with polarized light microscopy to identify monosodium urate (MSU) crystals, which has 84% sensitivity and 100% specificity. 1, 2

Essential Laboratory Tests

Synovial Fluid Analysis (Gold Standard)

• Joint aspiration with crystal analysis should be performed as the first diagnostic step for any patient with suspected gout, particularly when the diagnosis is uncertain, this is the first attack, or septic arthritis cannot be excluded. 3, 2
• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy. 2
• Gram stain and culture must be performed on all synovial fluid samples even when MSU crystals are identified, because gout and septic arthritis can coexist. 1, 3, 2
• Synovial fluid analysis can detect MSU crystals in approximately 70% of patients with known gout even during intercritical (pain-free) periods. 3, 2

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
• A cut-off of 6 mg/dL has moderate sensitivity (67%) and specificity (78%), with gender-specific cut-offs improving accuracy. 3
• Approximately 10% of patients with acute gout have normal serum uric acid levels during an attack because uric acid behaves as a negative acute phase reactant during inflammation. 1, 2
• Only 22% of asymptomatic patients with serum uric acid levels above 9 mg/dL develop gout over 5 years. 2
• Serum uric acid has limited diagnostic value during an acute attack but is essential for long-term management decisions. 1

Inflammatory Markers

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

Renal Function Tests

• Serum creatinine and estimated glomerular filtration rate (eGFR) should be measured to assess kidney function. 2
• 24-hour urinary uric acid excretion should be determined in selected patients, specifically those with family history of young-onset gout, onset of gout under age 25, or history of renal calculi. 1, 3

Risk Factor and Comorbidity Assessment

• Screen for modifiable risk factors and associated comorbidities including chronic kidney disease, obesity, hypertension, diabetes, dyslipidemia, ischemic heart disease, and heart failure. 3, 2
• Review medications that increase uric acid levels, particularly diuretics, low-dose aspirin, cyclosporine, and tacrolimus. 3, 2
• Assess dietary factors including alcohol consumption (especially beer), meat and shellfish intake, and high-fructose corn syrup consumption. 3, 2

Clinical Context for Laboratory Testing

• For typical recurrent podagra with hyperuricemia, clinical diagnosis is reasonably accurate (95% strength of recommendation) but not definitive without crystal confirmation. 1, 2
• When joint aspiration is technically difficult or unavailable, ultrasound showing the "double contour sign" (74% sensitivity, 88% specificity) or dual-energy CT (85-100% sensitivity, 83-92% specificity) can provide diagnostic confirmation without synovial fluid analysis. 3, 2, 4

Critical Pitfalls to Avoid

• Never diagnose gout based on hyperuricemia alone—up to 80% of patients with hyperuricemia never develop gout. 2
• Never exclude gout based on normal serum uric acid during an acute attack—approximately 10% of acute gout patients have normal levels. 2
• Always perform Gram stain and culture when obtaining synovial fluid, even if crystals are present, as crystal arthritis and septic arthritis can coexist. 1, 3, 2