Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (173 page)

   If an invasive intervention is considered, coagulation studies can be used to assess bleeding risk.

   If the prothrombin time is prolonged and other causes of coagulopathy are unlikely, chronic liver disease or hepatocellular etiologies become increasingly likely.

   Serum amylase would be obtained in cases where extrahepatic obstruction is suspected on the basis of history and physical examination.

   Diagnostic Imaging

   It is estimated that 25–40% of common bile duct obstructions are missed by both ultrasound and CT scanning. However, when intrahepatic cholestasis or hepatocellular etiologies are suspected, either of these noninvasive strategies is acceptable.

   
Ultrasound
: This is the least invasive and most inexpensive of the imaging procedures available to assess obstructive jaundice. Ultrasound determines the presence of obstructive jaundice by detecting dilated bile ducts.

   The sensitivity is 55–93%, and the specificity is 73–96%.

   False negatives are generally due to two factors:

   Inability to visualize the biliary tree (often secondary to interposed bowel gas)

   Absence of biliary dilation in the presence of obstruction

   It may be preferable, given its lower cost and radiation exposure.

   CT scanning is slightly more sensitive (74–96%) and specific (90–94%) than ultrasound in detecting the presence of biliary obstruction.

   A CT scan is more likely to show the site and cause of obstruction when compared with ultrasound.