How is gout diagnosed in an adult male with a history of joint pain and swelling, particularly in the big toe?

How Gout is Diagnosed

Synovial fluid analysis with identification of monosodium urate (MSU) crystals under polarized light microscopy is the definitive diagnostic test for gout and should be performed when clinical judgment indicates diagnostic testing is necessary. 1

Gold Standard Diagnostic Test

• Joint aspiration with synovial fluid analysis for MSU crystals is the reference standard for diagnosing acute gout, with sensitivity of 63-78% and specificity of 93-100% when performed by experienced operators. 2, 3, 4
• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarized light microscopy. 2, 3, 4
• Always perform Gram stain and culture even when MSU crystals are identified, because gout and septic arthritis can coexist. 2, 3, 4

When to Perform Joint Aspiration

• Perform arthrocentesis when septic arthritis cannot be clinically excluded, when the clinical presentation is atypical or uncertain, or when this is the patient's first suspected gout attack. 3, 4
• Joint aspiration is technically difficult in smaller joints and rarely performed in primary care settings where most gout is initially diagnosed. 3

Clinical Diagnosis Approach

When joint aspiration is not feasible or in typical presentations, clinical features can support the diagnosis:

High-Value Clinical Features

• Rapid onset of severe pain reaching maximum intensity within 6-24 hours has an odds ratio of 3.58 for gout. 2, 4
• Monoarticular involvement of the first metatarsophalangeal joint (podagra) is highly characteristic, with an odds ratio of 7.29. 2, 4
• Visible tophi on physical examination have an odds ratio of 7.29 for gout. 4
• Overlying erythema with severe tenderness over the affected joint strongly suggests gout. 2, 4
• History of previous similar acute episodes with complete resolution between attacks supports the diagnosis. 2

Clinical Algorithms

• Several validated clinical algorithms incorporating patient signs and symptoms have good specificity and sensitivity (>80%) for diagnosing gout compared with synovial fluid analysis. 1
• For typical recurrent podagra with hyperuricemia and characteristic features, clinical diagnosis has 95% strength of recommendation, though it is not definitive without crystal confirmation. 3
• The 2015 ACR/EULAR classification criteria provide a hierarchical scoring system with sensitivity of 92% and specificity of 89% (including imaging). 3

Critical Caveat

• The relatively low specificity of clinical algorithms raises the risk for missed septic arthritis, which has a similar initial presentation—do not skip joint aspiration when infection cannot be clinically excluded. 2, 3, 4

Laboratory Testing

Serum Uric Acid

• Measure serum uric acid level, but never diagnose gout based on hyperuricemia alone—only 22% of patients with uric acid >9 mg/dL develop gout within 5 years. 2, 3, 4
• Hyperuricemia alone has only 53-61% specificity for gout. 2, 3, 4
• Normal serum uric acid essentially rules out gout in the clinical context. 2, 3
• Approximately 10% of patients with acute gout have normal serum uric acid levels during an attack because uric acid behaves as a negative acute phase reactant during inflammation. 3

Additional Laboratory Tests

• Measure ESR and CRP at baseline for diagnosis and prognosis of inflammatory arthritis. 2
• Check renal function (creatinine, eGFR) to assess for chronic kidney disease, a critical risk factor with relative risk of 4.95. 2

Imaging Studies

Ultrasound (First-Line Advanced Imaging)

• Ultrasound should be the initial advanced imaging modality when joint aspiration is not feasible, with sensitivity of 74% and specificity of 88% for the "double contour sign" at cartilage surfaces. 2, 3, 4
• Detecting tophi appearing as hyperechoic masses with "wet clumps of sugar" appearance has sensitivity of 65% and specificity of 80%. 2
• Always scan the first metatarsophalangeal joint bilaterally, even if asymptomatic, as this is the disease-specific target site. 2
• Ultrasound performs better in late disease (>2 years duration) with 63% sensitivity compared to 50% in early disease (<2 years). 2

Dual-Energy CT (Second-Line Imaging)

• Use DECT when ultrasound is inconclusive and disease duration >2 years, with sensitivity of 85-100% and specificity of 83-92% for detecting MSU crystal deposits. 2, 3, 4, 5
• DECT provides specific color-coded images showing MSU crystal deposits, distinguishing them from other soft tissue pathology. 2
• DECT is less sensitive in patients with shorter history of flares (<2 years). 3

Plain Radiography

• Plain radiographs have limited value for diagnosing acute gout flares but can be useful for identifying chronic changes and ruling out other conditions. 3, 5
• Conventional radiography should not be the primary diagnostic imaging modality for gout. 2

When Imaging Can Replace Synovial Fluid Analysis

• When characteristic features of MSU crystal deposition are identified on ultrasound (double contour sign or tophi) or on DECT, synovial fluid analysis is not needed to confirm the diagnosis. 2

Risk Factors to Document

Document the following risk factors that support the diagnosis:

• Male sex, overweight/obesity (relative risk 3.81), hypertension (relative risk 3.93), chronic kidney disease (relative risk 4.95). 1, 2
• Diuretic use (relative risk 1.72), excess alcohol intake, diet rich in meat and seafood. 1, 2
• Associated comorbidities including diabetes, cardiovascular disease, and dyslipidemia. 3

Diagnostic Algorithm

Step 1: Assess clinical presentation—rapid onset severe pain, podagra, erythema, history of similar attacks. 2, 4

Step 2: If septic arthritis cannot be excluded OR diagnosis is uncertain OR first attack—perform joint aspiration with synovial fluid analysis for MSU crystals plus Gram stain and culture. 1, 3, 4

Step 3: If joint aspiration not feasible and typical presentation with hyperuricemia—clinical diagnosis is acceptable (95% strength of recommendation). 3

Step 4: If joint aspiration not feasible and diagnosis uncertain—perform ultrasound looking for double contour sign and tophi, always including first MTP joint bilaterally. 2, 3, 4

Step 5: If ultrasound inconclusive and disease duration >2 years—consider DECT. 2, 3

Common Pitfalls to Avoid

• Never diagnose gout based on hyperuricemia alone—this has only 53-61% specificity and up to 80% of patients with hyperuricemia never develop gout. 2, 3, 4
• Do not skip joint aspiration when septic arthritis cannot be clinically excluded—gout and septic arthritis can coexist. 2, 3, 4
• Do not rely on imaging alone—integrate patient history, physical examination, and laboratory findings. 2, 4
• Do not use plain radiographs as the primary diagnostic modality for acute gout. 2
• Ultrasound sensitivity is significantly lower in early disease (<2 years), with only 50% for double contour sign versus 63% in late disease. 2