What labs should be ordered when suspecting gout?

Laboratory Tests for Diagnosing Gout

When suspecting gout, the definitive diagnostic test is synovial fluid analysis for monosodium urate (MSU) crystals, while serum uric acid and other laboratory tests should be ordered to assess risk factors and guide management. 1, 2

Essential Diagnostic Tests

• Synovial fluid analysis: Examination under polarized light microscopy for MSU crystals is the gold standard for definitive diagnosis of gout, with extremely high specificity (100%) and good sensitivity (84%) 1, 2
• Gram stain and culture of synovial fluid should still be performed even when MSU crystals are identified, as gout and sepsis may coexist 3
• Serum uric acid (SUA): Essential for management but insufficient alone for diagnosis, as many patients with hyperuricemia never develop gout, and during acute attacks, serum levels may be normal 3, 1

Additional Laboratory Tests

• Complete blood count: To assess for inflammatory response and rule out other conditions 2
• Renal function tests (creatinine, BUN): Important for medication selection and dosing, especially for urate-lowering therapy 3, 1
• Liver function tests: Baseline measurement before initiating medications like allopurinol or febuxostat 1
• Renal uric acid excretion: Should be determined in selected patients, particularly those with:
  • Family history of young-onset gout
  • Onset of gout under age 25
  • History of renal calculi 3, 2

Assessment of Comorbidities and Risk Factors

• Metabolic panel: To evaluate for features of metabolic syndrome 2
• Lipid profile: To assess cardiovascular risk and hyperlipidemia 1, 2
• Fasting blood glucose: To screen for diabetes and insulin resistance 2
• Urinalysis: To evaluate for crystalluria and renal involvement 3, 2

Common Pitfalls to Avoid

• Relying solely on serum uric acid for diagnosis: Up to 35% of patients with acute gout attacks may have normal SUA levels (<420 μmol/L) due to increased renal excretion during acute inflammation 4, 5
• Failing to perform synovial fluid analysis: Clinical diagnosis alone is less accurate than crystal identification, especially in atypical presentations 3
• Overlooking assessment of cardiovascular risk factors: Gout is strongly associated with metabolic syndrome and cardiovascular disease 1, 2
• Not considering medication effects on uric acid levels: Certain medications (thiazides, loop diuretics, low-dose aspirin, niacin) can elevate serum urate levels 3

Diagnostic Algorithm

1. First step: Attempt synovial fluid aspiration and analysis for MSU crystals from affected joint 3, 2
2. Second step: Order serum uric acid, renal function, and liver function tests 3, 1
3. Third step: Assess for comorbidities with metabolic panel, lipid profile, and fasting glucose 2
4. Fourth step: Consider 24-hour urine collection for uric acid in patients with early-onset gout or renal stones 3
5. Fifth step: If synovial fluid analysis is not possible, use clinical criteria (rapid onset of severe pain, swelling, erythema, especially in first MTP joint) 3, 6

Remember that while radiographs may be useful for differential diagnosis in chronic gout, they are not useful for confirming early or acute gout 1.