Blood Test for Gout Diagnosis
Order a serum uric acid test, but understand that it cannot confirm or exclude gout—the definitive diagnosis requires synovial fluid aspiration with crystal identification under polarized light microscopy. 1
Primary Blood Test: Serum Uric Acid
The blood test to check for gout is serum uric acid (also called serum urate—these terms are interchangeable as uric acid exists predominantly as urate at physiologic pH). 2
Diagnostic Value and Critical Limitations
Hyperuricemia (>7 mg/dL in males, >6 mg/dL in females) has only 57% sensitivity and 92% specificity for gout, meaning it misses many cases and is present in many people who never develop gout. 3
The EULAR guidelines explicitly state that serum uric acid levels do not confirm or exclude gout (strength of recommendation: 95% CI 92-99), as many people with hyperuricemia never develop gout, and patients with crystal-proven gout may have normal levels during acute attacks. 1, 3, 2
Approximately 10% of people with gout have serum uric acid levels below 6 mg/dL during acute flares. 3
Why Serum Uric Acid Can Be Misleading During Acute Attacks
Serum uric acid behaves as a negative acute phase reactant, temporarily decreasing during acute inflammation and stress—the exact time when you're most likely to test it. 3, 4
Increased renal excretion of uric acid occurs during acute inflammatory episodes, causing levels to drop paradoxically when inflammation is present. 3, 4
For diagnostic purposes, serum uric acid should ideally be measured during the intercritical period (between attacks) rather than during acute flares, as levels are typically higher when inflammation has resolved. 3
The Gold Standard: Synovial Fluid Analysis
Demonstration of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates permits definitive diagnosis of gout with the highest strength of recommendation (96,95% CI 93-100). 2
A routine search for MSU crystals is recommended in all synovial fluid samples obtained from undiagnosed inflamed joints (strength of recommendation 90,95% CI 83-97). 2
Crystal identification from asymptomatic joints during intercritical periods allows definite diagnosis (strength of recommendation 84,95% CI 78-91). 2
Additional Laboratory Testing in Selected Cases
- 24-hour urinary uric acid excretion should be determined in selected patients, including those with:
- Family history of young-onset gout
- Gout onset before age 25
- History of renal calculi
- Consideration for uricosuric therapy 3
Clinical Context: Your Patient with Left Knee Pain
For your patient with left knee pain and risk factors (kidney stones, obesity):
Order serum uric acid, but recognize its limitations—a normal result does NOT exclude gout, especially during an acute attack. 1, 3
Strongly consider joint aspiration of the left knee to look for MSU crystals under polarized light microscopy—this is the only way to definitively diagnose gout. 2
If gout and septic arthritis are both suspected, perform Gram stain and culture even if MSU crystals are identified, as these conditions can coexist (strength of recommendation 93,95% CI 87-99). 2
Common Pitfalls to Avoid
Never exclude gout based solely on a normal serum uric acid level, particularly during an acute attack when levels are artificially lowered by inflammation. 1, 3, 4
Never diagnose gout based on hyperuricemia alone—only about 22% of asymptomatic patients with serum uric acid levels above 9 mg/dL develop gout over 5 years. 4
Don't rely on clinical diagnosis alone for typical presentations—while podagra and tophi have high diagnostic value (likelihood ratios of 30.64 and 39.95 respectively), crystal confirmation is still recommended for definitive diagnosis. 1