Which laboratory tests should be obtained when evaluating a patient with suspected gout?

Laboratory Tests for Suspected Gout

Essential Diagnostic Laboratory Tests

Joint aspiration with synovial fluid analysis for monosodium urate (MSU) crystals using polarized light microscopy is the definitive diagnostic test and should be performed whenever feasible, particularly when the diagnosis is uncertain, this is the first attack, or septic arthritis cannot be excluded. 1, 2

Synovial Fluid Analysis (Gold Standard)

• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy with 84% sensitivity and 100% specificity. 2
• Gram stain and culture must be performed on all synovial fluid samples even when MSU crystals are identified, because gout and septic arthritis coexist in up to 4% of cases. 2, 3
• Synovial fluid analysis can detect MSU crystals in approximately 70% of patients with known gout even during intercritical (pain-free) periods. 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
• 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. 2
• Only 22% of asymptomatic patients with serum uric acid levels above 9 mg/dL develop gout over 5 years. 1, 2
• Serum uric acid has 53-61% specificity for gout diagnosis but its absence makes gout highly improbable. 2

Inflammatory Markers

• Check erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) to assess inflammation severity and help rule out infection. 2
• Complete blood count should be obtained to evaluate for leukocytosis that may suggest septic arthritis. 2

Renal Function Assessment

• Measure serum creatinine and estimated glomerular filtration rate (eGFR) to assess kidney function, as chronic kidney disease is a major risk factor (relative risk 4.95) and influences treatment decisions. 2, 4

Specialized Laboratory Tests (Selected Patients)

24-Hour Urinary Uric Acid Excretion

• Measure 24-hour urinary uric acid excretion in patients with family history of young-onset gout, gout onset before age 25, or history of kidney stones to identify urate overproducers versus underexcretors. 1, 2
• Overexcretion is defined as >1000 mg/day on regular diet; underexcretion is defined as uric acid clearance <6 mL/min. 1
• This test influences the choice of urate-lowering therapy but is not required for diagnosis. 1

When Joint Aspiration Is Mandatory

• Arthrocentesis must be performed when septic arthritis cannot be clinically excluded, because missed infection carries substantial mortality and risk of limb loss. 2
• Joint aspiration is required for the first suspected gout attack in a patient without prior crystal-proven diagnosis. 2
• Aspiration is necessary in atypical presentations including polyarticular involvement, unusual joint distribution, or presence of fever. 2

Clinical Diagnosis Without Aspiration

• For typical recurrent podagra with hyperuricemia and characteristic features (rapid pain onset within 24 hours, erythema, male gender, prior similar attacks, no fever), clinical diagnosis has 95% strength of recommendation when aspiration is not feasible. 2
• Multiple validated clinical algorithms demonstrate sensitivities and specificities >80% for diagnosing gout compared to synovial fluid analysis. 1, 4
• The ACR/EULAR classification criteria achieve 92% sensitivity and 89% specificity when imaging is incorporated (score ≥8 points). 2

Risk Factor and Comorbidity Assessment

• Systematically screen for modifiable risk factors including chronic kidney disease (RR 4.95), obesity (RR 3.81), hypertension (RR 3.93), and diuretic use (RR 1.72). 2, 4
• Evaluate for associated comorbidities including renal impairment, ischemic heart disease, heart failure, diabetes mellitus, and dyslipidemia. 1, 2
• Assess medication use including thiazide and loop diuretics, low-dose aspirin, cyclosporine, and tacrolimus. 2
• Document alcohol consumption (particularly beer) and dietary patterns high in meat, shellfish, and high-fructose corn syrup. 2

Critical Pitfalls to Avoid

• Never diagnose gout based solely on hyperuricemia—many patients with elevated uric acid never develop gout, and 10% of acute gout patients have normal levels during attacks. 2, 4
• Never exclude septic arthritis based only on clinical appearance; aspiration with Gram stain and culture is mandatory even when MSU crystals are identified. 2, 3
• Do not delay joint aspiration when infection cannot be ruled out—coexistent gout and septic arthritis occurred in 73% of one case series with two deaths and one amputation. 2
• Serum uric acid measured during an acute flare may be falsely normal due to its behavior as a negative acute phase reactant. 2