Why do we order uric acid tests?

Why We Order Uric Acid Tests

Uric acid testing is primarily ordered to diagnose gout, evaluate kidney stone risk, monitor patients with certain kidney disorders, and assess patients with early-onset gout or urolithiasis for uric acid overproduction. 1

Primary Clinical Indications

1. Diagnosis and Management of Gout

• Diagnostic aid: While hyperuricemia is a major risk factor for gout, it has limitations as a standalone diagnostic test:

  • Serum uric acid (SUA) may be normal during acute gout attacks as it behaves as a negative acute phase reactant 1
  • Definitive diagnosis requires identification of monosodium urate (MSU) crystals from joint fluid 1
  • Hyperuricemia increases likelihood ratio for gout diagnosis, especially when using population-specific reference ranges 1

• Treatment monitoring: To assess efficacy of urate-lowering therapy (ULT)

  • Target SUA levels below 6 mg/dL to prevent gout flares and resolve tophi 1
  • Regular monitoring needed when using allopurinol, febuxostat, or other ULT 2

2. Evaluation of Kidney Stone Risk

• Ordered in patients with history of urolithiasis to assess risk of uric acid stones 1
• Helps identify patients who may benefit from urine alkalinization therapy 1

3. Assessment of Renal Function

• Monitoring patients with chronic kidney disease (CKD) as both conditions can affect each other 3
• Hyperuricemia may be both a marker and potential contributor to CKD progression 3

4. Evaluation of Uric Acid Metabolism Disorders

• Specifically recommended for patients with:
  • Gout onset before age 25 1
  • Family history of young-onset gout 1
  • History of kidney stones 1
  • These patients should be screened for uric acid overproduction via 24-hour urine collection 1

Special Considerations

Uricosuric Therapy Considerations

• The 2020 ACR guidelines conditionally recommend against checking urinary uric acid for patients considered for or receiving uricosuric treatment 1
• Standard practice dictates avoiding uricosurics in patients with known renal calculi or moderate-to-severe CKD (stage ≥3) 1

Asymptomatic Hyperuricemia

• Generally not an indication for treatment unless SUA >9 mg/dL with CKD stage ≥3 or urolithiasis 1
• May have value as a cardiovascular risk marker, though this remains under investigation 1, 4

Common Pitfalls in Uric Acid Testing

1. Timing of measurement during acute attacks:

   • SUA may be falsely normal or low during acute gout flares 1
   • Best measured during intercritical periods for diagnostic purposes

2. Overreliance on SUA for gout diagnosis:

   • Many people with high SUA never develop gout
   • Some patients with crystal-proven gout may have normal SUA at presentation 1

3. Failure to consider medications affecting SUA:

   • Thiazide and loop diuretics, niacin, and calcineurin inhibitors can elevate SUA 1
   • Low-dose aspirin (≤325 mg daily) has minimal clinical impact on SUA 1

4. Neglecting 24-hour urine collection in appropriate cases:

   • Important for distinguishing between uric acid overproduction and underexcretion 1
   • Critical for young patients with gout and those with urolithiasis 1

In summary, while uric acid testing is most commonly associated with gout diagnosis and management, it serves multiple important clinical purposes related to kidney function, stone risk, and metabolic disorders that affect morbidity, mortality, and quality of life.