What labs are recommended for diagnosing and managing gout?

Laboratory Tests for Gout

Definitive Diagnostic Test

Synovial fluid analysis with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard for definitive gout diagnosis, and should be performed whenever feasible, particularly when the diagnosis is uncertain or septic arthritis must be excluded. 1, 2

• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy 2
• Gram stain and culture must still be performed even when MSU crystals are identified, as gout and septic arthritis can coexist 2
• Tophus aspiration is an alternative source for crystal identification if joint aspiration is not feasible 1

Essential Laboratory Tests for Initial Evaluation

Serum Uric Acid (SUA)

• Critical caveat: Serum uric acid alone is insufficient for diagnosis because many patients with hyperuricemia never develop gout, and 63% of patients with acute gout attacks may have normal SUA levels during the flare 2, 3
• SUA is essential for ongoing management and monitoring treatment efficacy, not diagnosis 2
• Normal SUA during an acute attack correlates with increased inflammatory markers and urinary uric acid excretion 3

Renal Function Assessment

• Creatinine and estimated glomerular filtration rate (eGFR) must be measured in all patients with gout or hyperuricemia 2, 4
• Renal function determines medication selection and dosing, particularly for allopurinol (start ≤50 mg daily if CKD stage ≥3) 5
• Blood urea nitrogen (BUN) should also be assessed 2

Cardiovascular Risk Assessment

• Assessment of cardiovascular risk factors is recommended in all gout patients, as gout is associated with metabolic syndrome 1, 4
• Screen for hypertension, diabetes, hyperlipidemia, and obesity 5

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are elevated in 77% and 30% of acute gout cases respectively 3
• These markers help support the diagnosis when crystal identification is not possible 3

Specialized Laboratory Tests for Selected Patients

24-Hour Urine Uric Acid Collection

Indicated for: 2, 4, 5

• Gout onset before age 25
• Family history of young-onset gout
• History of urolithiasis (kidney stones)
• Consideration of uricosuric therapy

This test distinguishes uric acid overproducers from underexcretors, which influences treatment selection 2

Urinalysis

• Perform to evaluate for crystalluria and renal involvement 2
• Important for detecting concurrent kidney disease 2

Monitoring Tests During Treatment

Serum Uric Acid Monitoring

• Check SUA every 2-4 weeks during dose titration of urate-lowering therapy 2, 5
• Target SUA <6 mg/dL (360 μmol/L) for most patients 1, 2, 4
• Target SUA <5 mg/dL (300 μmol/L) for severe gout with tophi or frequent attacks 2, 4, 5
• Continue monitoring to ensure sustained target achievement 2

Liver Function Tests

• Baseline and periodic liver function tests are required for febuxostat therapy 2
• Monitor when using other urate-lowering agents as clinically indicated 2

Renal Function Monitoring

• Periodic monitoring is essential, especially when using allopurinol 4
• Adjust allopurinol dosing based on creatinine clearance 2

Common Pitfalls to Avoid

1. Relying solely on serum uric acid for diagnosis - This is the most common error, as SUA may be normal in up to 63% of acute attacks 2, 3

2. Failing to perform synovial fluid analysis when feasible - This leads to less accurate diagnosis, especially in atypical presentations 2

3. Not checking for medication effects - Thiazides, loop diuretics, low-dose aspirin, and niacin elevate serum urate levels and must be considered 2

4. Overlooking cardiovascular risk assessment - Gout patients have increased cardiovascular risk requiring systematic evaluation 1, 2

5. Inadequate monitoring during treatment - Regular SUA monitoring is essential to ensure target levels are reached and maintained 2

6. Ignoring drug interactions - Particularly important with allopurinol and azathioprine, mercaptopurine, or anticoagulants 2

Diagnostic Algorithm When Crystal Identification Not Possible

If synovial fluid analysis cannot be performed: 2

1. Assess for classical clinical features: podagra (first MTP joint involvement), rapid onset of severe pain/swelling/erythema, rapid response to colchicine 1, 2
2. Measure serum uric acid (recognizing its limitations) 2
3. Check inflammatory markers (ESR, CRP) 3
4. Evaluate renal function and cardiovascular risk factors 2, 4
5. Consider imaging (ultrasound for double contour sign, DECT for urate deposits) if available 1
6. Apply validated clinical criteria such as 2015 ACR/EULAR classification criteria (score ≥8 diagnostic) 1