What are the key clinical features and diagnostic criteria to differentiate gout arthritis in a patient, potentially with a history of similar episodes and risk factors such as obesity, hypertension, kidney disease, and diabetes?

Differentiating Gout Arthritis: Key Diagnostic Features

The definitive diagnosis of gout requires demonstration of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates via polarized light microscopy, and this should be attempted in every patient with suspected gout. 1

Three-Step Diagnostic Algorithm

The European League Against Rheumatism (EULAR) recommends a hierarchical approach 1, 2:

Step 1: Crystal Identification (Gold Standard)

• Perform joint aspiration with synovial fluid analysis in every patient with suspected gout, as MSU crystal demonstration provides definitive diagnosis 1
• MSU crystals appear as needle-shaped, negatively birefringent under polarized light microscopy 3, 4
• Sensitivity: 63-84%, Specificity: 93-100% when performed by experienced operators 1, 4
• Critical pitfall: Always perform Gram stain and culture even when MSU crystals are identified, as gout and septic arthritis can coexist 3, 4

Step 2: Clinical Diagnosis (When Aspiration Not Feasible)

Highly Suggestive Clinical Features 1, 2:

• Rapid onset of severe pain reaching maximum intensity within 6-24 hours (odds ratio 3.58) 4, 5
• Monoarticular involvement of first metatarsophalangeal joint (podagra) - highest diagnostic value with likelihood ratio of 30.64 and odds ratio of 7.29 2, 3, 4
• Overlying erythema with severe tenderness 1, 2
• Previous similar acute episodes - recurrent pattern strongly favors gout 1, 3
• Visible tophi on examination (likelihood ratio 39.95, odds ratio 7.29) 2, 3, 4
• Male gender 1
• Associated cardiovascular diseases 1
• Hyperuricemia (likelihood ratio 9.74 when present) 3

Important caveat: These features achieve >80% sensitivity and specificity only when multiple features are present together 4. Clinical diagnosis alone is insufficient for atypical presentations 2.

Step 3: Imaging (When Crystal Identification Not Possible)

Ultrasound (First-line imaging) 4:

• "Double contour sign" at cartilage surfaces: 74% sensitivity, 88% specificity 1, 2, 3, 4
• Detects tophi not evident on clinical examination 1

Dual-Energy CT (DECT) (Second-line) 4:

• 85-100% sensitivity, 83-92% specificity for MSU crystal deposits 2, 3, 4
• Less sensitive in early disease or shorter symptom duration 3

Plain Radiography 1:

• Limited value for acute gout diagnosis 1, 3
• Useful for chronic changes and ruling out alternative diagnoses 3

Critical Risk Factors to Document

Strongly Associated Comorbidities (EULAR Level 1a evidence) 1:

• Chronic kidney disease (relative risk 4.95) 2, 3
• Hypertension (relative risk 3.93) 3
• Obesity (relative risk 3.81) 2, 3
• Diabetes mellitus 1, 6
• Cardiovascular disease 1, 6

Medication-Induced Risk 1:

• Diuretics (relative risk 1.72) 1, 3
• Low-dose aspirin 1
• Cyclosporine, tacrolimus 1

Dietary/Lifestyle Factors 1:

• Excess alcohol consumption (particularly beer and spirits) 1, 3
• Diet high in meat and shellfish 1, 3
• Non-diet sodas 1

Laboratory Testing: What NOT to Do

Never diagnose gout based on hyperuricemia alone - this is explicitly contraindicated by EULAR guidelines (Level 2a evidence, Grade B recommendation) 1:

• Only 22% of patients with uric acid >9 mg/dL develop gout within 5 years 4
• 30-40% of patients with acute gout have normal serum uric acid during the attack 3, 4
• Hyperuricemia has only 53-61% specificity for gout 4

When to Strongly Suspect Gout vs. Other Conditions

Gout is highly likely when 1, 2:

• First MTP joint involvement (podagra) with rapid onset (<24 hours) 1, 2, 3
• Maximum pain within 6-12 hours 3, 5
• More than one previous similar attack 1, 3
• True joint involvement with joint line tenderness and restricted range of motion (not just bursitis) 3

Consider alternative diagnoses when 1, 2:

• Fever or systemic signs suggesting infection - must rule out septic arthritis 3
• Polyarticular presentation without typical features 2
• Absence of hyperuricemia in appropriate clinical context 4
• Atypical joint involvement without risk factors 2

Practical Clinical Algorithm

For typical recurrent podagra with hyperuricemia and no fever/systemic signs, clinical diagnosis is reasonably accurate (>80% sensitivity and specificity) 3. However, for atypical presentations or first episodes, synovial fluid analysis is mandatory to avoid missing septic arthritis 1, 2, 3.