Laboratory Tests for Gout Diagnosis and Management
For gout diagnosis, synovial fluid aspiration with polarized light microscopy to identify monosodium urate (MSU) crystals is the gold standard test you should perform first, while serum uric acid alone is insufficient for diagnosis but essential for ongoing management. 1, 2
Diagnostic Laboratory Tests
Primary Diagnostic Test
- Synovial fluid analysis is the reference standard—aspirate the affected joint and examine under polarizing microscopy for needle-shaped, negatively birefringent MSU crystals (specificity 100%, sensitivity 84%) 1, 2
- Perform Gram stain and culture on the same synovial fluid sample even when MSU crystals are identified, as gout and septic arthritis can coexist 1, 2
- Synovial fluid analysis can detect MSU crystals in approximately 70% of patients even during intercritical (pain-free) periods 2
Serum Uric Acid Testing
- Serum uric acid (SUA) measurement is essential for management but cannot diagnose gout alone—many patients with hyperuricemia never develop gout, and 63% of patients with acute gout attacks have normal SUA levels during the attack 1, 3
- The diagnostic cut-off of 6 mg/dL has only moderate sensitivity (67%) and specificity (78%) 2
- During acute attacks, inflammatory markers and increased urinary uric acid excretion can lower serum levels, creating false reassurance 3
Management and Monitoring Laboratory Tests
Baseline Assessment
- Renal function tests (creatinine, BUN, creatinine clearance)—critical for medication selection and dosing of urate-lowering therapy 1
- Liver function tests—baseline required before starting febuxostat, with periodic monitoring during treatment 1
- Urinalysis—evaluate for crystalluria and renal involvement 1
- Cardiovascular risk factors—assess lipid panel, glucose, blood pressure as gout strongly associates with metabolic syndrome 1
Special Circumstances Requiring 24-Hour Urine Collection
- Measure 24-hour urinary uric acid excretion in patients with: 1, 2
- Family history of young-onset gout
- Gout onset before age 25
- History of kidney stones (urolithiasis)
- Consideration for uricosuric therapy (probenecid, benzbromarone)
Ongoing Monitoring During Treatment
- Serum uric acid every 2-4 weeks during dose titration of urate-lowering therapy 1
- Target SUA <6 mg/dL (0.36 mmol/L) for most patients 1
- Target SUA <5 mg/dL (0.30 mmol/L) for severe gout (tophi, frequent attacks, chronic gouty arthritis) 1
- Periodic renal function monitoring, especially when using allopurinol (dose adjustment based on creatinine clearance) 1
- Periodic liver function tests when using febuxostat 1
Diagnostic Algorithm When Synovial Fluid Analysis Is Not Possible
If joint aspiration is not feasible, make a clinical diagnosis based on: 1, 2
- Rapid onset of severe pain, swelling, and erythema
- Monoarticular involvement, especially first metatarsophalangeal joint (podagra)
- Previous similar acute episodes
- Male gender
- Associated cardiovascular diseases
- Hyperuricemia (though not diagnostic alone)
- Ultrasound showing double contour sign (highly specific for urate deposits) 2
Critical Pitfalls to Avoid
- Never exclude gout based on normal serum uric acid during an acute attack—this occurs in the majority of acute presentations 1, 3
- Never rely on serum uric acid alone for diagnosis—hyperuricemia is neither sensitive nor specific for gout 1, 2
- Always consider medication effects on uric acid levels—thiazides, loop diuretics, low-dose aspirin, and niacin elevate SUA 1, 2
- Never skip synovial fluid culture even with confirmed MSU crystals—coexistent septic arthritis can occur 1, 2
- Check for drug interactions when using allopurinol with azathioprine, mercaptopurine, or anticoagulants 1