Elevation in all of the following laboratory studies would be indicative of liver disease EXCEPT:
B. Aspartate aminotransferase
C. Conjugated bilirubin
D. Unconjugated bilirubin
E. Urine bilirubin
The answer is D. (Chap. 358) It is important to understand the patterns of laboratory abnormalities that indicate liver disease is present. One way to consider laboratory evaluation of liver disease is to consider three general categories of tests: tests based on excretory function of the liver, tests of biosynthetic activity of the liver, and coagulation factors. The most common tests of liver function fall under the category of tests based on the detoxification and excretory function of the liver. These include serum bilirubin, urine bilirubin, ammonia, and enzyme levels. Bilirubin can exist as a conjugated and an unconjugated form. The unconjugated form is often referred to as the indirect fraction. Isolated elevation in the unconjugated form of bilirubin is typically not related to liver disease but is most commonly seen in hemolysis and a number of benign genetic conditions such as Gilbert syndrome. In contrast, conjugated hyperbilirubinemia almost always indicates disease of the liver or biliary tract. Conjugated bilirubin is water soluble and excreted in the urine, but unconjugated bilirubin is not. Rather, it binds to albumin in the blood. Therefore, bilirubinuria implies liver disease as well. Among the serum enzymes, it is useful to consider enzymes as those that are associated with hepatocellular injury or those that reflect cholestasis. Alanine and aspartate aminotransferases are the primary enzymes that indicate hepatocyte injury. Alkaline phosphatase is the most common enzyme elevated in cholestasis, but bone disease also causes increased alkaline phosphatase. In some cases, one needs additional information to determine whether the alkaline phosphatase is liver or bone in origin. Other tests that would be elevated in cholestatic liver disease are 5′-nucleotidase and γ-glutamyl transferase. The primary test of synthetic function is measurement of serum albumin. Coagulation factors can be directly measured, but impaired production of coagulation factors in liver disease is primarily inferred from elevations in prothrombin time.