In a patient with normal serum uric acid during acute gout flares, is a 24‑hour urinary uric acid test useful for diagnosis and management?

24-Hour Urinary Uric Acid Testing in Gout with Normal Serum Levels

A 24-hour urinary uric acid test is generally not beneficial for diagnosing gout when serum uric acid is normal during acute flares, but it should be obtained in specific clinical scenarios: patients under age 25 at gout onset, those with a family history of early-onset gout, or patients with a history of kidney stones. 1, 2, 3

Why Serum Uric Acid Can Be Normal During Gout Flares

Your normal serum uric acid during acute attacks does not exclude gout—this occurs in approximately 10-40% of patients with crystal-proven gout. 4, 5, 6

Key pathophysiologic mechanisms:

• Serum uric acid behaves as a negative acute phase reactant, meaning it temporarily drops during acute inflammation and stress 1, 2, 4
• Renal excretion of uric acid increases during acute inflammatory episodes, lowering blood levels 1, 3, 7
• Studies show 24-hour urinary uric acid excretion rises significantly during acute flares (from 4.68% to 5.54% fractional excretion), while serum levels fall 7
• One study found 63.3% of patients with acute gout had normal serum uric acid levels, with 55% showing increased urinary excretion 5

When 24-Hour Urine Testing IS Indicated

EULAR guidelines recommend measuring 24-hour urinary uric acid excretion in these specific situations (strength of recommendation: 72,95% CI 62-81): 1, 2, 3

• Age under 25 years at gout onset
• Family history of young-onset gout
• History of kidney stones (renal calculi)
• Consideration for uricosuric therapy (medications that increase uric acid excretion)

Clinical interpretation of results:

• Overexcretion: >1000 mg/24 hours (>6 mmol/24 hours) indicates excessive uric acid production—guides use of xanthine oxidase inhibitors (allopurinol, febuxostat) rather than uricosuric agents 3
• Normal excretion with hyperuricemia: Indicates decreased renal clearance 3
• Hypoexcretion: Uric acid clearance <6 mL/min 3

What You Should Do Instead for Diagnosis

The definitive diagnosis requires joint aspiration with synovial fluid analysis—serum or urine uric acid levels alone cannot confirm or exclude gout (EULAR strength of recommendation: 95% CI 92-99). 2, 4

Diagnostic algorithm:

1. Joint aspiration during an acute attack or even between attacks (crystals persist in previously affected joints) 4
2. Polarized light microscopy to identify needle-shaped, negatively birefringent monosodium urate crystals 4
3. Concurrent Gram stain and culture because gout and septic arthritis can coexist 4
4. Measure serum uric acid between attacks (intercritical period) for baseline assessment and treatment monitoring—not for diagnosis 2, 4

Clinical Features That Support Gout Despite Normal Uric Acid

High-probability clinical indicators: 4

• Podagra (first toe joint involvement): 96% sensitivity, 97% specificity
• Rapid-onset severe monoarticular pain peaking within 6-12 hours with overlying erythema
• Recurrent self-limited attacks with complete resolution between episodes
• Visible tophi on examination (highly specific)

Critical Pitfalls to Avoid

• Do not exclude gout based solely on normal serum uric acid during an acute attack—this is a common diagnostic error 2, 4
• Do not measure uric acid excretion during an acute attack for classification purposes, as renal excretion transiently increases during flares 3, 7
• Do not diagnose gout based only on hyperuricemia—specificity is only 53-61%, and many hyperuricemic individuals never develop gout 4
• Recurrence risk is not lower in patients with normal serum uric acid during attacks compared to those with elevated levels 6

Treatment Monitoring Considerations

Once gout is confirmed, target serum uric acid <6 mg/dL (or <5 mg/dL if tophi present) measured during intercritical periods for treatment monitoring—not diagnosis. 1, 2, 4