What labs are used to determine uric acid levels in patients, especially those with a history of gout, kidney stones, or undergoing chemotherapy?

Laboratory Testing for Uric Acid Determination

Order a serum uric acid test (also called serum urate) using standard enzymatic methods, but measure it during the intercritical period (between attacks) rather than during acute flares for diagnostic purposes, as levels frequently drop to normal during acute inflammation. 1

Standard Laboratory Test

• Serum uric acid is the primary laboratory test, measured using enzymatic methods (uricase-catalase or uricase-peroxidase) that are widely available in clinical laboratories 2
• The test can be ordered simply as "uric acid" or "serum uric acid" and will be universally understood by laboratories 3
• Serum uric acid and serum urate are identical measurements, as uric acid exists predominantly as urate at physiologic pH, so either term is acceptable 3

Critical Timing Considerations

• Measure serum uric acid between attacks (intercritical period) for diagnostic purposes, not during acute gout flares, as approximately 10% of gout patients have levels below 6 mg/dL during acute attacks 1
• Serum uric acid behaves as a negative acute phase reactant, temporarily decreasing during acute inflammation due to increased renal excretion 1, 4
• The European League Against Rheumatism states that serum uric acid levels do not confirm or exclude gout—many people with hyperuricemia never develop gout, and patients with crystal-proven gout may have normal levels during acute attacks (strength of recommendation: 95% CI 92-99) 1, 3

Interpretation Guidelines

• Normal reference ranges: 3.5-7.2 mg/dL in adult males and postmenopausal women; 2.6-6.0 mg/dL in premenopausal women 5
• Target for treatment: <6 mg/dL (360 μmol/L) for most patients, or <5 mg/dL (300 μmol/L) in severe gout with tophi 1, 3
• Hyperuricemia (male >7 mg/dL, female >6 mg/dL) has sensitivity of 57% and specificity of 92% for clinical gout 6

Additional Testing in Selected Patients

• 24-hour urinary uric acid excretion should be determined in patients with:

  • Family history of young-onset gout 6
  • Gout onset before age 25 6
  • History of renal calculi 6
  • Consideration for uricosuric therapy 6

• Renal function monitoring (BUN, serum creatinine, or creatinine clearance) should be performed periodically in patients with decreased renal function, hypertension, or diabetes mellitus, as these conditions affect uric acid excretion 7

Monitoring During Treatment

• Measure serum uric acid every 2-5 weeks during dose titration of urate-lowering therapy to ensure target levels are achieved and maintained 4, 3
• Use serum uric acid as an index for determining correct dosage and schedule for maintaining levels within the normal range 7

Common Pitfalls to Avoid

• Do not rely on serum uric acid alone for diagnosis—the gold standard remains identification of monosodium urate crystals in synovial fluid or tophus aspirates 4, 3
• Do not exclude gout based on normal uric acid levels during an acute attack, as this has a markedly low negative likelihood ratio 4
• Do not measure during acute flares for diagnostic purposes, as inflammation artificially lowers levels 1, 4
• Be aware that dietary factors (high purine foods, fructose, alcohol), medications (thiazide diuretics), and hydration status can significantly fluctuate serum uric acid levels 2