Diagnostic Tests for Gout
The gold standard for diagnosing gout is the identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates using polarized light microscopy. 1, 2
Primary Diagnostic Approach
- Synovial fluid analysis: Joint aspiration with synovial fluid analysis for MSU crystals is the reference standard for diagnosing gout, with sensitivity and specificity approaching 100% when performed by experienced operators 2
- MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy 2
- Even when MSU crystals are identified, Gram stain and culture should still be performed as gout and septic arthritis may coexist 2
When Joint Aspiration Is Not Feasible
If joint aspiration cannot be performed due to patient discomfort, lack of experienced clinician, or unavailability of polarizing microscope, clinical judgment may be appropriate, especially in less ambiguous situations 1:
- Clinical algorithms: Several clinical algorithms have demonstrated good sensitivity and specificity (>80%) compared to synovial fluid MSU crystal analysis 1, 3
- Clinical algorithms have higher sensitivity in patients with longer disease duration (>2 years) than in those with recent-onset symptoms 3
- No single clinical algorithm is currently recommended over others, but several promising algorithms show sensitivities and specificities greater than 80% 1
Clinical Features Highly Suggestive of Gout
- Podagra: Monoarticular arthritis affecting the first metatarsophalangeal joint is highly characteristic of gout 2, 3
- Rapid onset: Development of severe pain reaching maximum intensity within 24 hours 2, 4
- Erythema: Presence of redness over the affected joint 2
- Previous attacks: History of similar episodes 2
- Male gender: Significantly higher risk in men (relative risk 7.64) 3
- Cardiovascular comorbidities: Associated conditions like hypertension and heart disease 2
Imaging Studies
Ultrasound: Sensitivity ranges from 37% to 100% and specificity from 68% to 97% for detecting MSU crystal deposition 2, 3
- The "double contour sign" on ultrasound has pooled sensitivity of 74% and specificity of 88% for detecting tophi 2
Dual-Energy CT (DECT): Sensitivity ranges from 85% to 100% and specificity from 83% to 92% for detecting MSU crystal deposition 2, 3
- Less sensitive in patients with recent-onset acute gout 3
Plain radiographs: Limited value for diagnosing acute gout flares, but useful for identifying chronic changes and ruling out other conditions 2
Laboratory Tests
- Serum uric acid: Hyperuricemia alone is not diagnostic of gout, as many people with elevated serum uric acid never develop gout 3
- The diagnosis of gout should not be made on the presence of hyperuricemia alone 1
Risk Factor Assessment
- Medical conditions: Chronic kidney disease, obesity, hypertension, heart disease, diabetes, and dyslipidemia 2
- Medications: Diuretics, low-dose aspirin, cyclosporine, tacrolimus 2
- Dietary factors: Alcohol consumption (particularly beer and spirits), diet high in meat and shellfish, non-diet sodas 2
Common Pitfalls to Avoid
- Relying solely on serum urate levels: Hyperuricemia is not diagnostic on its own 1, 3
- Failing to consider septic arthritis: Gout and septic arthritis may coexist; always perform Gram stain and culture when aspirating joints 2
- Misdiagnosing pseudogout: Calcium pyrophosphate crystals (pseudogout) can be mistaken for gout; proper crystal identification is essential 5
- Overlooking recent-onset gout: Clinical algorithms have lower sensitivity in patients with recent-onset symptoms compared to those with established disease 3
When clinical presentation is typical (such as podagra with appropriate risk factors and no signs of infection), a clinical diagnosis may be reasonable without joint aspiration 1, but whenever possible, crystal identification remains the definitive diagnostic approach.