Which investigations should be ordered for a patient with suspected gouty arthritis?

Diagnostic Studies for Suspected Gouty Arthritis

Joint Aspiration with Synovial Fluid Analysis (Gold Standard)

Joint aspiration with polarized light microscopy to identify needle-shaped, negatively birefringent monosodium urate (MSU) crystals is the definitive diagnostic test and should be performed whenever feasible, particularly for first attacks, atypical presentations, or when septic arthritis cannot be excluded. 1

• Synovial fluid analysis has sensitivity and specificity approaching 100% when performed by experienced operators 1
• MSU crystals appear as needle-shaped, negatively birefringent crystals under polarizing microscopy 1
• Gram stain and culture must be performed on all aspirated fluid even when MSU crystals are identified, because gout and septic arthritis can coexist 1
• Joint aspiration is technically difficult and painful in smaller joints, which limits routine use in primary care settings 2

Serum Uric Acid Level

• Measure serum uric acid, but recognize its significant limitations: hyperuricemia alone cannot diagnose gout, and normal levels do not exclude it 1
• 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 1
• Only 22% of asymptomatic patients with serum uric acid levels above 9 mg/dL develop gout over 5 years 1
• Serum uric acid has 53-61% specificity for gout diagnosis but is essential for long-term management decisions 1

Imaging Studies

First-Line: Ultrasound

• Ultrasound should be the initial advanced imaging modality when joint aspiration is not feasible or when clinical diagnosis is uncertain 1
• The "double contour sign" on ultrasound has 74% sensitivity and 88% specificity for detecting MSU crystal deposition 1
• Ultrasound detects tophi with 65% sensitivity and 80% specificity 1
• Always include the first metatarsophalangeal joint in imaging, even if asymptomatic, as this is the disease-specific target site 1
• Ultrasound performs better in late disease (>2 years duration) with 63% sensitivity compared to 50% in early disease (<2 years) 1

Second-Line: Dual-Energy CT (DECT)

• DECT should be considered when ultrasound is negative but clinical suspicion remains high, particularly in patients with disease duration >2 years 1
• DECT has 85-100% sensitivity and 83-92% specificity for detecting MSU crystal deposition 1
• DECT is less sensitive in early disease and involves radiation exposure and higher cost 1

Limited Role: Plain Radiography

• Plain radiographs are useful only for assessing chronic structural damage or excluding alternative diagnoses 1
• Radiography should not be the primary diagnostic imaging modality for acute gout 1
• Radiographs are frequently normal in early or acute gout, showing at most nonspecific soft-tissue swelling 1

Inflammatory Markers

• Check ESR and CRP to assess inflammation severity and help rule out infection 1
• Complete blood count should be obtained to evaluate for infection 1

Renal Function Assessment

• Measure serum creatinine and calculate eGFR to assess kidney function 1
• Chronic kidney disease increases gout risk 4.95-fold and influences treatment decisions 1

24-Hour Urinary Uric Acid Excretion (Selected Patients Only)

• Obtain 24-hour urinary uric acid in patients with family history of young-onset gout, onset under age 25, or history of renal calculi 1
• This test identifies urate overproducers versus underexcretors, which influences choice of urate-lowering therapy 1

Risk Factor and Comorbidity Screening

• Systematically screen for modifiable risk factors: chronic kidney disease (RR 4.95), obesity (RR 3.81), hypertension (RR 3.93), and diuretic use (RR 1.72) 1
• Evaluate medications including diuretics, low-dose aspirin, cyclosporine, and tacrolimus 1
• Screen for associated comorbidities: renal impairment, ischemic heart disease, heart failure, diabetes mellitus, and dyslipidemia 1
• Assess alcohol consumption and dietary factors (meat, shellfish, high-fructose corn syrup) 1

Clinical Diagnostic Algorithm When Aspiration Not Feasible

• Clinical algorithms demonstrate sensitivities and specificities >80% for diagnosing early-onset gout, particularly in patients with recent-onset attacks 2
• Key clinical features include: maximum inflammation developing within 24 hours, erythema over the affected joint, first metatarsophalangeal joint involvement (podagra), and history of previous similar attacks 1
• However, the relatively low specificity of clinical algorithms raises risk for missed diagnosis of septic arthritis—if infection cannot be excluded clinically, joint aspiration with Gram stain and culture is mandatory 2

Common Pitfalls to Avoid

• Do not diagnose gout based on hyperuricemia alone—up to 80% of patients with hyperuricemia never develop gout 1
• Do not exclude gout based on normal serum uric acid during an acute attack 1
• Septic arthritis cannot be excluded based solely on clinical appearance; when in doubt, aspirate 1
• Do not rely on imaging alone—integrate patient history, physical examination, and laboratory findings into the diagnostic assessment 1