American Board of Internal Medicine (ABIM) Certification Practice Exam

What should be measured in patients with elevated alkaline phosphatase levels and no other abnormal liver chemistry results?

1. Serum bilirubin level

2. Serum gamma-glutamyl transferase level

3. Serum aspartate aminotransferase level

4. Serum creatinine level

The correct answer is: Serum gamma-glutamyl transferase level

When evaluating patients with elevated alkaline phosphatase levels and no other abnormal liver chemistry results, measuring the serum gamma-glutamyl transferase (GGT) level is particularly important. GGT is an enzyme that is primarily found in the liver and is sensitive to liver pathology. It plays a crucial role in the differentiation of the source of alkaline phosphatase elevation. If both alkaline phosphatase and GGT are elevated, it typically indicates a hepatic origin of the alkaline phosphatase increase, which may point towards conditions such as cholestasis or bile duct obstruction. Conversely, if alkaline phosphatase is elevated while GGT is normal, it is more likely that the elevation is due to a non-hepatic source, such as bone disease. The other measurements, while useful in different contexts, do not provide the same level of specificity for identifying the source of the alkaline phosphatase elevation. Serum bilirubin levels are generally assessed when there's a suspicion of jaundice or hemolysis, and serum aspartate aminotransferase (AST) levels are more relevant in cases of liver injury. Serum creatinine levels relate to kidney function and are not directly pertinent to interpreting abnormalities in alkaline phosphatase. Thus, GGT is