What lab tests are recommended for the diagnosis and management of gout?

Laboratory Tests for Gout Diagnosis and Management

Diagnostic Laboratory Tests

Synovial fluid analysis with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard for definitive gout diagnosis, and serum uric acid measurement is essential for ongoing management and treatment monitoring. 1, 2

Synovial Fluid Analysis

• Aspirate the affected joint and examine synovial fluid under polarized light microscopy to identify needle-shaped, negatively birefringent MSU crystals 1, 2
• Perform Gram stain and culture of synovial fluid even when MSU crystals are identified, as gout and septic arthritis may coexist 1
• This test provides definitive diagnosis and should be attempted as the first diagnostic step whenever possible 1

Serum Uric Acid (SUA)

• Measure SUA but do not rely on it alone for diagnosis, as many patients with hyperuricemia never develop gout, and up to 30% of patients with acute gout have normal SUA levels during an attack 1, 2
• SUA is essential for management decisions and monitoring treatment efficacy, not for initial diagnosis 1

Baseline Laboratory Assessment

• Check renal function tests (creatinine, BUN, creatinine clearance) at baseline, as these are critical for medication selection and dosing, especially for urate-lowering therapy 1, 3
• Perform liver function tests at baseline, particularly if considering febuxostat therapy 1
• Order urinalysis to evaluate for crystalluria and renal involvement 1

Specialized Testing for Selected Patients

• Obtain 24-hour urine collection for uric acid excretion in patients with:
  • Family history of young-onset gout 1
  • Onset of gout under age 25 1
  • History of renal calculi 1

Management and Monitoring Laboratory Tests

Urate-Lowering Therapy Targets

• Target SUA below 6 mg/dL (0.36 mmol/L) for most patients 1, 2
• Target SUA below 5 mg/dL (0.30 mmol/L) for patients with severe gout (tophi, frequent attacks, chronic gouty arthritis) 1, 2

Monitoring During Treatment

• Check SUA every 2-4 weeks during dose titration of urate-lowering therapy to ensure target levels are reached 1, 2
• Monitor renal function periodically, as allopurinol dosing must be adjusted based on creatinine clearance 1, 3
• Perform periodic liver function tests during febuxostat therapy 1
• Reassess prothrombin time periodically in patients receiving dicumarol who are given allopurinol 3

Cardiovascular and Metabolic Assessment

• Screen all gout patients for cardiovascular risk factors and metabolic syndrome components, as these are commonly associated with gout 1, 2

Common Pitfalls to Avoid

• Do not diagnose or exclude gout based solely on serum uric acid levels, as SUA may be normal during acute attacks and hyperuricemia alone does not confirm gout 1, 2
• Do not fail to perform synovial fluid analysis in atypical presentations, as this leads to less accurate diagnosis 1
• Consider medication effects on uric acid levels, particularly thiazides, loop diuretics, low-dose aspirin, and niacin, which can elevate serum urate 1
• Do not overlook drug interactions when prescribing allopurinol, especially with azathioprine, mercaptopurine (requiring dose reduction to one-third to one-fourth), or anticoagulants 1, 3
• Adjust allopurinol dosing in renal impairment: patients with severely impaired renal function may require only 100 mg per day or 300 mg twice weekly 3

Diagnostic Algorithm

1. First: Attempt synovial fluid aspiration and analysis for MSU crystals from the affected joint 1
2. Second: Order serum uric acid, renal function, and liver function tests 1
3. Third: Perform Gram stain and culture of synovial fluid 1
4. Fourth: Consider 24-hour urine collection for uric acid in patients with early-onset gout or renal stones 1
5. Fifth: Use clinical criteria (rapid onset of severe pain, swelling, erythema, especially in the first MTP joint) if synovial fluid analysis is not possible 1