What labs are recommended to diagnose and manage gout?

Laboratory Tests for Gout Diagnosis and Management

Definitive Diagnostic Test

Synovial fluid aspiration with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard for diagnosing gout and should be performed whenever feasible, particularly when septic arthritis cannot be excluded. 1, 2

• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy with sensitivity and specificity of 85-100% and 83-92%, respectively 1, 2
• A trained operator and polarizing microscope are required for accurate crystal identification 3, 2
• Gram stain and culture must still be performed even when MSU crystals are identified, as gout and septic arthritis can coexist 1

Essential Laboratory Tests for Management

Serum Uric Acid (SUA)

• Serum uric acid is essential for ongoing management and monitoring treatment efficacy, but is insufficient alone for diagnosis 1
• Many patients with hyperuricemia never develop gout, and 30-40% of patients with acute gout have normal SUA levels during an attack 1
• Target SUA <6 mg/dL (0.36 mmol/L) for most patients, or <5 mg/dL (0.30 mmol/L) for severe gout with tophi or frequent attacks 1
• Check SUA every 2-4 weeks during urate-lowering therapy dose titration 1

Renal Function Tests

• Measure serum creatinine and calculate creatinine clearance at baseline and periodically 1
• Renal function is critical for medication selection and dosing, particularly for allopurinol and uricosuric agents 1
• Adjust allopurinol dose based on creatinine clearance 1

Liver Function Tests

• Baseline and periodic liver function tests are required for febuxostat therapy 1

Additional Laboratory Tests for Selected Patients

24-Hour Urine Uric Acid Excretion

• Determine renal uric acid excretion in patients with:
  • Family history of young-onset gout 1
  • Onset of gout before age 25 1
  • History of renal calculi 1

Urinalysis

• Perform urinalysis to evaluate for crystalluria and renal involvement 1

Cardiovascular Risk Assessment

• Screen for cardiovascular risk factors and comorbidities in all patients with gout or hyperuricemia 4, 1
• This includes assessment for obesity, dyslipidemia, hypertension, and metabolic syndrome 3

Diagnostic Algorithm When Synovial Fluid Analysis Is Not Possible

If joint aspiration is not feasible or yields negative results:

1. Apply the 2015 ACR/EULAR classification criteria (score ≥8 is diagnostic) with sensitivity of 92% and specificity of 89% with imaging, or 85% sensitivity and 78% specificity without imaging 3

2. Consider clinical features with highest diagnostic value:

   • Presence of tophi (odds ratio 7.29) 3
   • Podagra (first metatarsophalangeal joint involvement) 4, 3
   • Rapid onset with maximal inflammation within 24 hours 3
   • Resolution of pain within 2 weeks (odds ratio 3.58) 3
   • Erythema over the affected joint 3

3. Consider advanced imaging if diagnosis remains uncertain:

   • Dual-energy CT (DECT): sensitivity 85-100%, specificity 83-92% 1, 3, 2
   • Ultrasonography with "double contour" sign: pooled sensitivity 74%, specificity 88% 3

Critical Pitfalls to Avoid

• Never rely solely on serum uric acid for diagnosis - SUA may be normal during acute attacks in up to 40% of cases 1
• Do not overlook medication effects - thiazides, loop diuretics, low-dose aspirin, and niacin can elevate serum urate levels 1
• Always consider septic arthritis - perform Gram stain and culture even when MSU crystals are present 1
• Do not fail to assess renal function - essential for safe medication dosing and identifying patients at risk for complications 1
• Monitor for drug interactions - particularly allopurinol with azathioprine, mercaptopurine, or anticoagulants 1