Synovial Fluid Uric Acid Levels
The question about "how high" uric acid is in synovial fluid is clinically misleading—what matters for gout diagnosis is not a specific numeric threshold but rather the presence of monosodium urate (MSU) crystals visualized under polarized light microscopy, which is the gold standard for definitive diagnosis. 1
The Critical Distinction: Crystals vs. Uric Acid Concentration
The provided guidelines emphasize that demonstration of MSU crystals in synovial fluid permits definitive diagnosis of gout with the highest strength of recommendation (96,95% CI 93-100). 1 The focus in clinical practice should be on crystal identification, not on measuring absolute uric acid concentrations in synovial fluid.
When Synovial Fluid Uric Acid Measurement May Be Helpful
While not the standard approach, research suggests that synovial fluid uric acid levels can provide diagnostic information when polarized microscopy is unavailable:
Synovial Fluid to Serum Uric Acid Ratio (SSR)
- A synovial fluid to serum uric acid ratio (SSR) ≥1.01 has 89.6% sensitivity and 66.3% specificity for identifying gout, making it a potential aid when crystal analysis is not available. 2
- This ratio is significantly higher in gout patients compared to other arthritides including osteoarthritis, pseudogout, rheumatoid arthritis, and ankylosing spondylitis. 2
Crystal Persistence in Synovial Fluid
- MSU crystals persist in synovial fluid of asymptomatic joints in 97% of previously inflamed knees in untreated gout patients, demonstrating that crystals remain detectable between acute attacks. 3
- In joints that have never been inflamed, only 22% (8 of 37) contained MSU crystals, highlighting the importance of aspirating previously affected joints. 3
Important Clinical Caveats
Serum Uric Acid Has Limited Correlation
- Serum uric acid levels do not confirm or exclude gout, as many people with hyperuricemia never develop gout, and patients with crystal-proven gout may have normal serum levels during acute attacks. 1
- Serum uric acid behaves as a negative acute phase reactant, temporarily decreasing during acute inflammation. 1, 4, 5
The Diagnostic Standard Remains Crystal Identification
- 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). 1
- Crystal identification from asymptomatic joints during intercritical periods allows definite diagnosis (strength of recommendation 84,95% CI 78-91). 1
Coexisting Conditions
- Gout and septic arthritis may coexist, so Gram stain and culture should still be performed even if MSU crystals are identified when infection is suspected (strength of recommendation 93,95% CI 87-99). 1
Practical Approach
Rather than focusing on numeric uric acid values in synovial fluid:
Aspirate the joint and examine for MSU crystals under polarized light microscopy—this is the definitive diagnostic test. 1
If polarized microscopy is unavailable, measure both synovial fluid and serum uric acid to calculate the SSR, with values ≥1.01 supporting gout diagnosis. 2
Consider aspiration of previously affected joints during asymptomatic periods, as crystals persist in 97% of these joints in untreated patients. 3
Do not rely on serum uric acid levels alone for diagnosis, as they have limited diagnostic value especially during acute attacks. 1, 4