Use of several screening tests improves the detection of hepatobiliary abnormalities, helps differentiate the basis for clinically suspected disease, and determines the severity of liver disease. Many tests are available, but relatively few improve patient care. Clinical laboratories usually select a battery of automatic analyses. The prothrombin time, done on request, indicates the severity of hepatocellular disease. Serum bilirubin may not be a particularly sensitive index of liver disease or prognosis, but it is established and necessary.
Sophisticated techniques can separate the conjugates of bilirubin but add nothing of clinical value.
An early sign of hepatobiliary disease can be bilirubinuria, which develops in acute viral hepatitis even before jaundice appears. It may be absent, however, under other circumstances despite increased serum bilirubin. Failure of bilirubin excretion into the small intestine reduces urobilinogen formation so that the urine may test falsely low or absent. Thus urobilinogen, though sensitive for mild liver disease, is too nonspecific and difficult to interpret. Alkaline phosphatases are a group of isoenzymes with a common capacity to hydrolyze organic phosphatase ester bonds in an alkaline medium, generating an organic radical and an inorganic phosphate.