Blood Tests for Gout Diagnosis
While serum uric acid (SUA) testing is commonly used in gout evaluation, it cannot definitively confirm or exclude gout diagnosis as levels may be normal during acute attacks and many people with hyperuricemia never develop gout. 1
Gold Standard Diagnostic Test
- Definitive diagnosis requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates examined under polarized light microscopy 1, 2
- A routine search for MSU crystals is strongly recommended in all synovial fluid samples obtained from undiagnosed inflamed joints 1
- Crystal identification from asymptomatic joints may allow definite diagnosis during intercritical periods 1
Role of Serum Uric Acid Testing
Limitations of SUA Testing
- SUA levels do not confirm or exclude gout 1, 2
- Many people with hyperuricemia never develop gout 1
- During acute attacks, SUA levels may be normal in up to 14-63% of patients 1, 3, 4
- SUA behaves as a negative acute phase reactant, being temporarily lowered during episodes of acute inflammation 1
SUA Testing Parameters
- A cut-off level of 6 mg/dl (360 μmol/L) has moderate sensitivity (0.67) and specificity (0.78) 1
- A higher cut-off of 7 mg/dl (420 μmol/L) has reduced sensitivity (0.57) but increased specificity (0.92) 1
- Gender-specific reference ranges improve diagnostic accuracy (LR increases from 3.00 to 7.61) 1
- Most laboratories calculate their own cut-off level as mean plus two standard deviations from their local healthy population (calculated separately for men and women) 1
Additional Laboratory Tests
Renal uric acid excretion should be determined in selected gout patients, especially those with:
- Family history of young-onset gout
- Onset of gout under age 25
- Renal calculi 1
Inflammatory markers may be elevated during acute attacks:
Clinical Implications
- General practitioners often diagnose gout without SUA testing (in ~40% of cases), leading to diagnostic uncertainty 5
- When SUA testing is not performed, diagnoses are often questionable or sometimes incorrect 5
- Even with normal SUA levels, acute gout attacks can still occur due to persistence of tophi and increased body uric acid pool 4
- SUA levels are typically higher in acute gout (mean 8.4 mg/dL) compared to asymptomatic hyperuricemia (mean 7.2 mg/dL) 6
Monitoring and Treatment Considerations
- For patients on urate-lowering therapy, the treatment target is SUA below 6 mg/dL (0.36 mmol/L) 1, 7
- Regular monitoring of SUA levels is essential for treatment efficacy assessment 1
- In patients with tophi, a more stringent target of <5 mg/dL (0.30 mmol/L) may be recommended for faster resolution 2
Important Caveats
- Gout and sepsis may coexist, so when septic arthritis is suspected, Gram stain and culture of synovial fluid should still be performed even if MSU crystals are identified 1, 2
- Risk factors for gout and associated comorbidities should be assessed, including features of metabolic syndrome (obesity, hyperglycemia, hyperlipidemia, hypertension) 1, 2
- Diagnostic accuracy improves when combining SUA testing with clinical features and imaging findings 1