Laboratory Tests for Diagnosing Gout
Synovial fluid aspiration with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard laboratory test for diagnosing gout, and should be performed whenever possible in patients with suspected acute gout. 1, 2, 3
Primary Diagnostic Test
Synovial fluid analysis is the reference standard and should be your first-line diagnostic approach: 1, 2
- Aspirate fluid from the affected joint and examine under polarizing microscopy for needle-shaped, negatively birefringent MSU crystals 2, 3
- This test has sensitivity and specificity of 85-100% and 83-92%, respectively 3
- Requires a trained operator and polarizing microscope to properly identify crystals 3
- Critical pitfall: Always perform Gram stain and culture on the same fluid sample, as gout and septic arthritis can coexist 2
Serum Uric Acid Testing
Serum uric acid (SUA) measurement is essential for management but insufficient for diagnosis alone: 2, 4
- Many patients with hyperuricemia never develop gout 2
- Major pitfall: SUA levels may be normal during acute gout attacks, so a normal level does NOT rule out gout 2, 4
- SUA is primarily used to guide urate-lowering therapy, with target levels <6 mg/dL (0.36 mmol/L) for most patients, or <5 mg/dL (0.30 mmol/L) for severe gout with tophi 2
Additional Laboratory Tests for Initial Workup
When evaluating a patient with suspected gout, order these tests: 2
- Renal function tests (creatinine, BUN): Essential for medication selection and dosing of urate-lowering therapy 2
- Liver function tests: Baseline testing before initiating febuxostat or other urate-lowering agents 2
- Urinalysis: Evaluate for crystalluria and renal involvement 2
Specialized Testing for Select Patients
24-hour urine collection for uric acid excretion should be obtained in: 2
- Patients with family history of young-onset gout
- Gout onset before age 25
- History of renal calculi (kidney stones)
When Synovial Fluid Analysis Is Not Possible
If joint aspiration cannot be performed or is technically difficult: 4, 3
- Use the 2015 ACR/EULAR classification criteria (score ≥8 points is diagnostic, with 92% sensitivity and 89% specificity) 4
- Consider advanced imaging: Dual-energy CT (DECT) has 85-100% sensitivity and 83-92% specificity 4, 3
- Ultrasonography can detect the "double contour" sign with 74% sensitivity and 88% specificity 4
Important Clinical Considerations
Medication review is critical when interpreting SUA levels: 2
- Thiazide diuretics, loop diuretics, low-dose aspirin, and niacin can all elevate serum urate levels 2
- Consider these confounders when assessing laboratory results
Cardiovascular risk assessment should be performed in all patients with gout or hyperuricemia, as these conditions are associated with increased cardiovascular disease 2