What labs and tests are used to diagnose and manage gout?

Laboratory Tests for Diagnosis and Management of Gout

The definitive diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirate through polarized light microscopy, which is the gold standard diagnostic test. 1, 2

Diagnostic Tests

• Synovial fluid analysis is the reference standard for gout diagnosis, with MSU crystals appearing as needle-shaped, negatively birefringent crystals under polarizing microscopy 3, 1
• Serum uric acid (SUA) level is essential for management but insufficient alone for diagnosis, as many patients with hyperuricemia never develop gout, and some patients with acute gout may have normal SUA levels during an attack 1, 2
• Gram stain and culture of synovial fluid should be performed even when MSU crystals are identified, as gout and sepsis may coexist 1
• Renal function tests (creatinine, BUN) are important for medication selection and dosing, especially for urate-lowering therapy 1, 4

Alternative Diagnostic Methods When Joint Aspiration Is Not Possible

• Dual-Energy Computed Tomography (DECT) shows high sensitivity (85-100%) and specificity (83-92%) for gout diagnosis, though it is less sensitive in patients with recent-onset acute gout 5, 2
• Ultrasonography demonstrates variable sensitivity (37-100%) and specificity (68-97%), with pooled sensitivity of 74% and specificity of 88% 5, 2
• Clinical prediction tools have demonstrated good sensitivity and specificity (>80%) compared to synovial fluid MSU crystal analysis 5, 2

Management and Monitoring Tests

• The target for urate-lowering therapy is SUA below 6 mg/dL (0.36 mmol/L) for most patients, or below 5 mg/dL (0.30 mmol/L) for patients with severe gout (tophi, frequent attacks) 1
• Regular monitoring of SUA during treatment is essential to ensure target levels are reached and maintained 1
• For patients on allopurinol, periodic liver function tests are recommended, particularly during the early stages of therapy 4
• For patients on febuxostat, baseline and periodic liver function tests are recommended 1
• For patients on uricosuric agents (e.g., probenecid), baseline renal function assessment is necessary 1

Special Considerations for Specific Patient Populations

• Renal uric acid excretion should be determined in selected patients, particularly those with:

  • Family history of young-onset gout 1
  • Onset of gout under age 25 1, 3
  • History of renal calculi 3, 1

• Evaluation of cardiovascular risk factors is recommended in all patients with gout or hyperuricemia 1

Common Pitfalls in Laboratory Testing for Gout

• Relying solely on serum uric acid for diagnosis, as SUA may be normal during acute attacks, and hyperuricemia alone does not confirm gout 1, 2
• Failing to perform synovial fluid analysis can lead to less accurate diagnosis, especially in atypical presentations 1
• Overlooking the need to check for drug interactions, particularly when using allopurinol with drugs like azathioprine, mercaptopurine, or anticoagulants 1, 4
• Not considering the effect of medications (thiazides, loop diuretics, low-dose aspirin, niacin) that can elevate serum urate levels 1, 3
• Inadequate monitoring of SUA during treatment, which is necessary to ensure target levels are reached and maintained 1

Diagnostic Algorithm

1. Attempt synovial fluid aspiration and analysis for MSU crystals from the affected joint 1, 2
2. Order serum uric acid, renal function, and liver function tests 1
3. Consider 24-hour urine collection for uric acid in patients with early-onset gout or renal stones 1
4. If synovial fluid analysis is not possible, use clinical criteria and alternative imaging methods like DECT or ultrasonography 1, 5
5. Evaluate for comorbidities and risk factors that may affect gout management 1, 5