When assessing elevated Liver Function Tests (LFTs), is an abdominal Computed Tomography (CT) scan or Ultrasound (US) better?

Ultrasound is the Preferred Initial Imaging for Elevated LFTs

Abdominal ultrasound should be the first-line imaging modality for evaluating elevated liver function tests (LFTs) due to its accessibility, safety, high specificity, and reasonable diagnostic accuracy. 1, 2

Rationale for Ultrasound as First-Line Imaging

Ultrasound offers several advantages as the initial imaging choice:

• High diagnostic accuracy: Sensitivity of 65-95% and positive predictive value of 98% for detecting liver parenchymal disease 1, 2
• Good detection of biliary obstruction: Specificity of 71-97% for biliary obstruction 1
• Safety profile: No radiation exposure, non-invasive
• Cost-effectiveness: Less expensive than CT or MRI
• Accessibility: Widely available in most clinical settings

The American College of Radiology (ACR) and American College of Gastroenterology explicitly recommend ultrasound as the initial diagnostic test of choice in patients with suspected liver disease or biliary obstruction 1, 2.

Limitations of Ultrasound

While ultrasound is the preferred initial test, it has important limitations:

• Variable sensitivity for biliary obstruction: 32-100% sensitivity range 1
• Limited detection of CBD stones: Only 22.5-75% sensitivity for common bile duct stones 1
• Visualization challenges: Subhepatic common duct may be obscured by bowel gas 1
• Reduced performance in obesity: Less effective in obese patients 2
• Limited fibrosis assessment: Cannot accurately stage fibrosis 2

When to Consider CT After Ultrasound

CT should be considered as a second-line imaging modality when:

1. Moderate to severe aminotransferase elevation: CT abdomen with IV contrast is appropriate following ultrasound 1
2. Suspected complications: When cholangitis, cholecystitis, or pancreatitis are suspected 1
3. Inconclusive ultrasound: When ultrasound findings are equivocal 1

CT offers superior spatial resolution with modern multidetector CT (MDCT) technology showing >90% sensitivity for biliary obstruction 1. CT is also very accurate for diagnosis and staging of pancreaticobiliary malignancies that may present with biliary obstruction 1.

When MRI/MRCP is Preferred Over CT

MRI with MRCP is superior to CT in specific scenarios:

• Persistent ALP elevation with negative ultrasound: MRI with MRCP is recommended 1
• Suspected biliary obstruction: MRI with MRCP is most useful for evaluating etiology 1
• Need for detailed biliary evaluation: MRI provides better visualization of both intra- and extrahepatic bile ducts 1

MRI has slightly better accuracy than CT for diagnosis and staging of pancreaticobiliary malignancies (90.7% vs. 85.1%) 1.

Algorithmic Approach to Imaging for Elevated LFTs

1. Initial imaging: Abdominal ultrasound for all patients with elevated LFTs

2. If hepatocellular pattern predominates (elevated ALT/AST):

   • For mild elevation: Ultrasound alone may be sufficient
   • For moderate/severe elevation: Add CT abdomen with IV contrast if ultrasound is inconclusive 1

3. If cholestatic pattern predominates (elevated ALP/GGT):

   • Ultrasound first
   • If ultrasound shows biliary dilation: Proceed to MRI with MRCP
   • If ultrasound is negative but ALP remains elevated: Consider MRI with MRCP 1

4. If hyperbilirubinemia is present:

   • Ultrasound is appropriate initial imaging
   • MRI with MRCP or CT with IV contrast are equivalent alternatives if further imaging is needed 1

Common Pitfalls to Avoid

• Relying solely on LFT severity: The yield of abdominal imaging correlates only weakly with both the severity and extent of abnormal LFTs 3
• Overuse of CT as initial imaging: Exposes patients to unnecessary radiation when ultrasound may be sufficient
• Underestimating ultrasound limitations: Be aware that ultrasound may miss small lesions and has reduced sensitivity for mild steatosis 2
• Routine repeat imaging: Serial LFT assessments following initial abnormalities are not always necessary in immunocompetent patients with subclinical derangement 4

By following this evidence-based approach, clinicians can optimize the diagnostic workup of patients with elevated LFTs while minimizing unnecessary testing and radiation exposure.