What are the sensitivity and specificity of musculoskeletal ultrasound (including the double‑contour sign, tophus detection, and snow‑storm sign) for diagnosing acute gout in an adult presenting with acute mono‑articular arthritis?

Sensitivity and Specificity of Ultrasound in Acute Gout

Musculoskeletal ultrasound demonstrates moderate to high diagnostic accuracy for acute gout, with the double-contour sign showing pooled sensitivity of 74% and specificity of 88%, though performance varies significantly by disease duration and specific ultrasound features assessed. 1

Overall Ultrasound Performance

Ultrasonography shows overall sensitivities ranging from 37% to 100% and specificities from 68% to 97%, depending on which ultrasound signs are assessed and whether evaluation is person-based or joint-based. 1

Specific Ultrasound Features

Double-Contour Sign (Most Clinically Useful)

• Pooled sensitivity: 74% (CI 52-88%) 1
• Pooled specificity: 88% (CI 68-96%) 1
• The double-contour sign appears as a hyperechoic, irregular enhancement over the hyaline cartilage surface 1
• Individual study sensitivity ranges from 37% (attributed to very early-onset gout) to 78%, with specificity ranging from 64% to 97% 1
• In one high-quality study, the double-contour sign was detected in 92% of crystal-proven gouty joints and in none of the control joints (P < 0.001) 2

Tophus Detection

• Person-based pooled sensitivity: 56% (CI 52-60%) 3
• Person-based pooled specificity: 94% (CI 92-96%) 3
• Joint-based pooled sensitivity: 48% (CI 40-57%) 3
• Joint-based pooled specificity: 96% (CI 91-99%) 3
• Tophi appear as hypoechoic to hyperechoic, inhomogeneous material surrounded by a small anechoic rim 2

Snowstorm Sign (Least Useful)

• Pooled sensitivity: 31% (CI 27-36%) 3
• Pooled specificity: 91% (CI 88-93%) 3
• This feature is ranked lowest for overall diagnostic accuracy among ultrasound findings 4

Critical Performance Modifiers

Disease Duration Significantly Affects Sensitivity

• Ultrasound sensitivity is markedly lower in recent-onset acute gout 1
• Sensitivity improves substantially in patients with disease duration >2 years compared to early disease 5
• The double-contour sign sensitivity drops to as low as 37% in very early-onset gout 1

Combined Ultrasound Features Improve Accuracy

• Simultaneous consideration of multiple ultrasound features (double-contour sign, tophi, snowstorm) yields 80% sensitivity and 83% specificity 3
• This combined approach provides better diagnostic performance than relying on any single ultrasound feature 3

Comparison to Other Imaging Modalities

• Dual-energy CT (DECT) demonstrates superior sensitivity (85-100%) and comparable specificity (83-92%) compared to ultrasound 1
• However, DECT is also less sensitive in patients with shorter history of flares 1
• Network meta-analysis ranks DECT highest for overall accuracy, followed by ultrasound double-contour sign, though differences in overall accuracy between these modalities are not statistically significant 4

Practical Clinical Algorithm

When to Use Ultrasound

• Ultrasound should be the first-line imaging modality when joint aspiration is not feasible or when clinical diagnosis is uncertain 5
• Always scan the first metatarsophalangeal joint bilaterally, even if asymptomatic, as this is the disease-specific target site 5

When Ultrasound May Be Insufficient

• In patients with disease duration <2 years, ultrasound sensitivity is substantially reduced (approximately 50% for double-contour sign) 5
• If ultrasound is negative but clinical suspicion remains high—particularly in patients with disease duration >2 years—consider DECT 5

Common Pitfalls to Avoid

• Do not rely on ultrasound alone in early acute gout (<2 years duration), as sensitivity is markedly reduced 1, 5
• Ultrasound must be performed and interpreted by trained operators to ensure accuracy 5
• Do not use ultrasound findings in isolation—integrate with patient history, physical examination, and laboratory findings 5
• When characteristic ultrasound features (double-contour sign or tophi) are identified, synovial fluid analysis is not needed to confirm diagnosis 5, 6
• However, if septic arthritis cannot be clinically excluded, joint aspiration with Gram stain and culture is mandatory regardless of ultrasound findings 5

Evidence Quality Considerations

Six studies evaluated ultrasound diagnostic accuracy, with four rated as good quality and two as moderate quality, all conducted in academic rheumatology departments 1. The American College of Physicians systematic review rated the strength of evidence for ultrasonography as low, noting that recency of first attack affects test performance 1.