Order Serum Uric Acid (Not "Urate")
Order serum uric acid—this is the standard laboratory test name used in clinical practice, though "urate" and "uric acid" refer to the same substance biochemically. 1
Understanding the Terminology
- Serum uric acid and serum urate are the same measurement—uric acid exists predominantly as urate at physiologic pH, so laboratories may use either term interchangeably. 1
- Most clinical laboratories in the United States report this test as "serum uric acid" or "uric acid, serum," so ordering "uric acid" will be universally understood. 1
Critical Diagnostic Limitations
Do not rely on serum uric acid levels alone to diagnose or exclude gout—this is a common pitfall that leads to misdiagnosis. 2
- Many people with hyperuricemia (>6.8 mg/dL) never develop gout, and patients with crystal-proven gout may have normal serum uric acid levels during acute attacks. 2
- Serum uric acid behaves as a negative acute phase reactant, temporarily decreasing during acute inflammation when you're most likely to be evaluating the patient. 2
The Definitive Diagnostic Test
Joint aspiration with synovial fluid analysis for monosodium urate (MSU) crystals under polarized light microscopy is the gold standard for diagnosis, not serum uric acid measurement. 2
- The European League Against Rheumatism gives demonstration of MSU crystals the highest strength of recommendation (96,95% CI 93-100) for definitive diagnosis. 2
- The American College of Rheumatology recommends routine search for MSU crystals in all synovial fluid samples from undiagnosed inflamed joints (strength of recommendation 90,95% CI 83-97). 2
When to Order Serum Uric Acid
Order serum uric acid for monitoring treatment targets, not for initial diagnosis:
- The treatment target is serum uric acid <6 mg/dL (360 μmol/L), and <5 mg/dL (300 μmol/L) in severe gout with tophi. 1
- Monitor serum uric acid levels regularly while titrating urate-lowering therapy to achieve and maintain target levels. 1
- The goal is to keep levels below the saturation point for monosodium urate (6.8 mg/dL) to dissolve existing crystals and prevent new crystal formation. 3
Additional Uric Acid Testing
Consider 24-hour urinary uric acid excretion only in specific circumstances:
- Order this test for patients with gout onset before age 25, family history of early-onset gout, or history of renal stones. 4
- Hyperexcretion is defined as >1000 mg/24h under a normal diet. 4
- This helps distinguish uric acid overproducers (who benefit from xanthine oxidase inhibitors) from underexcretors (who may respond to uricosurics). 4
- Never measure urinary uric acid during an acute gout attack, as renal excretion increases transiently during acute episodes. 4