What is the sensitivity of ultrasound (USG) for detecting common bile duct (CBD) stones?

Sensitivity of Ultrasound for Common Bile Duct Stones

Transabdominal ultrasound has a limited sensitivity of 73% (range 44-90%) for detecting common bile duct (CBD) stones, making it inadequate as a standalone diagnostic test for CBD stones. 1

Diagnostic Accuracy of Ultrasound for CBD Stones

Transabdominal ultrasound is commonly used as the first imaging test for suspected biliary pathology due to its wide availability, but it has significant limitations in detecting CBD stones:

• Sensitivity: 73% (range 44-90%)
• Specificity: 91% (range 84-95%)
• Performance varies based on pre-test probability:
  • Low pre-test probability (9.5%): Positive post-test probability 45%, Negative post-test probability 3%
  • Intermediate pre-test probability (40.8%): Positive post-test probability 85%, Negative post-test probability 17%
  • High pre-test probability (65.8%): Positive post-test probability 94%, Negative post-test probability 37% 1

Factors Affecting Ultrasound Sensitivity

Several factors influence the sensitivity of ultrasound for CBD stone detection:

• Stone size: Smaller stones (<8mm) are more likely to be missed 2
• CBD diameter: Stones are more difficult to detect in dilated bile ducts (≥12mm) 2
• Operator experience: Experienced examiners achieve significantly higher sensitivity (82%) compared to less experienced examiners (46%) 3
• Technical limitations: Gas in the duodenum and obesity can obscure visualization of the distal CBD

Comparison with Other Imaging Modalities

When compared to other diagnostic modalities for CBD stones:

• MRCP: Superior sensitivity of 93% (range 87-96%) and specificity of 96% (range 90-98%) 1, 4
• EUS: Excellent sensitivity of 95% (range 91-97%) and specificity of 97% (range 94-99%) 1, 4
• ERCP: Gold standard for diagnosis and treatment, but invasive with 4-5.2% risk of major complications 1

In a Cochrane systematic review, at a median pre-test probability of 41% for CBD stones:

• EUS post-test probabilities: 96% for positive test, 3% for negative test
• MRCP post-test probabilities: 94% for positive test, 5% for negative test 4

Clinical Implications and Recommendations

Given the limited sensitivity of ultrasound for CBD stones:

1. Initial evaluation: Transabdominal ultrasound remains appropriate as a first-line imaging test for suspected biliary pathology due to its wide availability, non-invasive nature, and ability to demonstrate biliary dilatation 1

2. Further testing: If CBD stones are suspected despite negative ultrasound:

   • For intermediate probability: MRCP or EUS is recommended
   • For high probability: Direct ERCP or surgical exploration may be warranted 1, 5

3. Risk stratification: Consider pre-test probability based on clinical, laboratory, and initial imaging findings:

   • High risk: CBD stone on ultrasound, features of cholangitis, or triad of pain, duct dilation, and jaundice in patients with gallstones
   • Intermediate risk: CBD dilatation with normal LFTs or abnormal LFTs with normal caliber biliary system
   • Low risk: Normal results 1, 5

Pitfalls and Caveats

• A normal ultrasound does not exclude CBD stones, especially small stones
• CBD dilatation on ultrasound (>8mm) should raise suspicion for obstruction even if stones are not visualized 1
• Ultrasound sensitivity is particularly poor in the emergency setting, with reported sensitivity as low as 25-63% for CBD stone detection 1
• Relying solely on ultrasound may lead to missed diagnoses, especially in patients with small stones or dilated bile ducts

In conclusion, while transabdominal ultrasound is a useful initial screening tool for biliary pathology, its limited sensitivity for CBD stones means that additional imaging with MRCP or EUS is often necessary when clinical suspicion remains despite negative ultrasound findings.