Blood Test to Rule Out Gout
Serum uric acid (SUA) cannot reliably rule out gout and should not be used as the primary test to exclude this diagnosis. 1
Why Serum Uric Acid Fails as a Rule-Out Test
The critical limitation: Many patients with crystal-proven gout have normal SUA levels at presentation, particularly during acute attacks. 1
- SUA behaves as a negative acute phase reactant, meaning it temporarily drops during acute inflammation and stress 1, 2
- Studies demonstrate that SUA levels are often lower during acute gout flares compared to intercritical periods 1, 2
- The mechanism involves increased renal excretion of uric acid during acute episodes 1, 2
- A normal SUA during an acute attack does not exclude gout 1, 2
The Gold Standard for Diagnosis
Demonstration of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates is the only definitive test to confirm or exclude gout. 1
- MSU crystals appear needle-shaped with strong negative birefringence under polarized microscopy 3
- This test has the highest strength of recommendation (96,95% CI 93-100) from EULAR guidelines 1
- Joint aspiration should be performed routinely in all undiagnosed inflamed joints to search for MSU crystals 1
- Crystal identification can be performed even during asymptomatic intercritical periods from previously affected joints 1
Clinical Diagnosis When Aspiration Is Not Feasible
For typical presentations (recurrent podagra with hyperuricemia), clinical diagnosis is reasonably accurate but never definitive without crystal confirmation. 1
Key clinical features highly suggestive of gout: 1
- Rapid development of severe pain, swelling, and tenderness reaching maximum within 6-12 hours
- Overlying erythema
- Podagra (first metatarsophalangeal joint involvement) has exceptional diagnostic value with likelihood ratio of 30.64 1
The Limited Role of Serum Uric Acid
While hyperuricemia is the most important risk factor for developing gout, SUA levels neither confirm nor exclude the diagnosis. 1
- Many people with hyperuricemia never develop gout (only ~22% with SUA >9 mg/dL develop gout over 5 years) 2
- Conversely, patients with crystal-proven gout may have normal SUA levels 1, 2
- Different cut-off values apply for men versus women, with men typically having higher baseline levels 1
- SUA measurement is most useful between attacks, not during acute presentations 1, 2
Critical Pitfall to Avoid
Do not rely on serum uric acid to rule out gout. 1, 2 The diagnostic uncertainty created by omitting crystal confirmation leads to questionable or incorrect diagnoses in a substantial proportion of cases. 4 In primary care settings, over 40% of gout diagnoses are made without SUA testing, and when clinical diagnosis occurs without crystal confirmation, diagnostic accuracy is significantly compromised. 4
When Septic Arthritis Is Suspected
Gout and sepsis can coexist, so Gram stain and culture of synovial fluid must still be performed even if MSU crystals are identified. 1 This is a critical safety consideration with high strength of recommendation (93,95% CI 87-99). 1