What labs are needed to rule out gout?

Laboratory Tests for Diagnosing Gout

The definitive diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates using polarized light microscopy, while serum uric acid measurement is essential for ongoing management but insufficient alone for diagnosis. 1, 2

Primary Diagnostic Tests

• Synovial fluid analysis with polarized light microscopy is the gold standard for gout diagnosis, identifying characteristic needle-shaped, negatively birefringent MSU crystals 2, 1
• Serum uric acid (SUA) level should be measured but is not diagnostic alone as many patients with hyperuricemia never develop gout, and some patients with acute gout may have normal SUA levels during an attack 2, 3
• Gram stain and culture of synovial fluid should still be performed even when MSU crystals are identified, as gout and sepsis may coexist 2, 1

When Synovial Fluid Analysis Is Not Possible

If synovial fluid analysis cannot be performed, a clinical diagnosis can be supported by:

• Monoarticular involvement of a foot (especially the first metatarsophalangeal joint) or ankle 2, 4
• Previous similar acute arthritis episodes 2
• Rapid onset of severe pain and swelling (reaching maximum within <24 hours) 2
• Erythema over the affected joint 2
• Male gender and associated cardiovascular diseases 2, 4
• Presence of hyperuricemia 2

Additional Laboratory Tests

• Renal function tests (creatinine, BUN) are important for medication selection and dosing, especially for urate-lowering therapy 1
• Renal uric acid excretion (24-hour urine collection) should be determined in selected patients, particularly those with:
  • Family history of young-onset gout 2, 1
  • Onset of gout under age 25 2, 1
  • History of renal calculi 2, 1

Assessment for Comorbidities

• Systematic assessment for associated comorbidities is recommended, including:
  • Obesity 2
  • Renal impairment 2
  • Hypertension 2
  • Ischemic heart disease 2
  • Heart failure 2
  • Diabetes 2
  • Dyslipidemia 2

Risk Factors for Hyperuricemia

• Chronic kidney disease 2
• Overweight 2
• Medications (including diuretics, low-dose aspirin, cyclosporine, tacrolimus) 2, 1
• Consumption of excess alcohol (particularly beer and spirits) 2
• Non-diet sodas 2
• Diet rich in meat and shellfish 2

Common Pitfalls to Avoid

• Relying solely on serum uric acid for diagnosis, as SUA may be normal during acute attacks (found in up to 63.3% of patients with acute gout) 1, 3
• Failing to perform synovial fluid analysis when possible, leading to less accurate diagnosis, especially in atypical presentations 1
• Not considering that gout and sepsis may coexist, necessitating both crystal examination and culture of synovial fluid 2
• Not accounting for the effect of medications that can elevate serum urate levels when interpreting results 1
• Making a gout diagnosis without testing SUA, which occurs in approximately 40% of cases in primary care and leads to diagnostic uncertainty 5

Diagnostic Algorithm

1. Attempt synovial fluid aspiration and analysis for MSU crystals from the affected joint (gold standard) 1, 6
2. Order serum uric acid, renal function, and liver function tests 1
3. If synovial fluid analysis is not possible, use clinical criteria and laboratory findings 2
4. Consider 24-hour urine collection for uric acid in patients with early-onset gout or renal stones 1
5. If diagnosis remains uncertain, consider imaging (ultrasound or dual-energy CT) to search for MSU crystal deposition 2