What is the most appropriate initial investigation for a 38-year-old woman with epigastric pain, vomiting, and multiple small gallstones, presenting with abdominal distention, epigastric tenderness, and abnormal laboratory results, including elevated alkaline phosphatase, amylase, direct bilirubin, and total bilirubin levels?

Most Appropriate Initial Investigation for Suspected Biliary Obstruction

Transabdominal ultrasound is the most appropriate initial investigation for this patient with suspected biliary obstruction due to gallstones. 1, 2

Clinical Presentation Analysis

This 38-year-old woman presents with classic symptoms suggestive of biliary pathology:

• Epigastric pain radiating to the back for 6 days
• Frequent vomiting
• Known multiple small gallstones
• Abdominal distention and epigastric tenderness
• Left pleural effusion on chest X-ray
• Laboratory values within normal range (alkaline phosphatase, amylase, direct and total bilirubin)

Diagnostic Approach

First-Line Investigation: Transabdominal Ultrasound

• Transabdominal ultrasound is the recommended initial imaging modality for suspected biliary obstruction according to multiple recent guidelines 1, 2
• The 2024 Infectious Diseases Society of America guidelines specifically recommend abdominal ultrasound as the initial diagnostic imaging modality for suspected acute cholecystitis or cholangitis 1
• Ultrasound offers several advantages:
  • High sensitivity (81%) and specificity (83%) for detecting gallstones 2
  • Non-invasive and widely available 1
  • No radiation exposure 1
  • Cost-effective 1
  • Can detect biliary dilatation, gallbladder wall thickening, and pericholecystic fluid 2

Key Ultrasound Findings to Look For

• Presence and location of gallstones (gallbladder, cystic duct, common bile duct)
• Biliary ductal dilatation
• Gallbladder wall thickening (>3mm)
• Pericholecystic fluid
• Sonographic Murphy's sign (pain when probe presses on gallbladder)

Why Other Options Are Less Appropriate

1. Urine analysis for amylase level (Option B):

   • Not indicated as the serum amylase is already normal (149 IU/L)
   • Urinary amylase has limited diagnostic value when serum levels are normal 3

2. Abdominal computed tomography (Option C):

   • Recommended as a second-line investigation only if ultrasound is equivocal 1
   • Involves radiation exposure 1
   • More costly than ultrasound 1
   • The 2024 IDSA guidelines specifically recommend CT only after an equivocal ultrasound 1

3. Analysis of gastrointestinal aspirated fluids (Option D):

   • Not a standard diagnostic approach for suspected biliary pathology
   • Invasive procedure with limited diagnostic yield in this clinical scenario
   • No guideline support for this as an initial investigation 1

Diagnostic Algorithm

1. Initial evaluation: Transabdominal ultrasound to assess for gallstones, biliary dilatation, and signs of inflammation 2

2. If ultrasound is equivocal/non-diagnostic:

   • Proceed to CT with IV contrast 1
   • Consider MRCP if biliary obstruction is still suspected 1, 2

3. If both ultrasound and CT are inconclusive:

   • Consider MRI/MRCP or HIDA scan 1

Important Considerations

• Despite normal laboratory values, this patient may still have biliary pathology, as demonstrated in case reports of atypical presentations 4
• The presence of known gallstones increases the pre-test probability of biliary disease 2
• Left pleural effusion can be associated with pancreatobiliary disease 3
• Early diagnosis is crucial to prevent complications such as cholangitis, pancreatitis, or gallbladder perforation 2

Ultrasound findings will guide subsequent management decisions, including the potential need for ERCP, cholecystectomy, or further imaging studies.