Laboratory Tests

Abstract

Key conceptsr Biochemical tests can be used to detect acute injury to the liver and chronic diseases of the liver before development of symptoms.r Elevations in serum levels of aminotransferases (aspartate aminotransferase and alanine aminotransferase) reflect hepatocellular injury and/or permeability of the liver cell membrane, but do not correlate with the extent of fibrosis or measures of metabolic functions of the liver.r An elevation of serum alkaline phosphatase occurs in response to intrahepatic cholestasis, extrahepatic obstruction of bile ducts, or infiltration of the liver with granulomatous inflammation, amyloidosis, or venous congestion.r The rate-limiting step in metabolism and disposition of bilirubin is the transport of conjugated bilirubin from the hepatocyte into the bile.An elevation in the conjugated (direct-reacting) fraction of bilirubin is evidence of intrahepatic cholestasis or extrahepatic obstruction of the bile ducts.r Serum levels of albumin may be low in patients with acute or chronic liver disease, but very often drop in response to the "acute phase response," making it difficult to evaluate hepatic metabolic function in clinical situations such as infection.r Although single laboratory tests are generally not useful in evaluating the severity of acute or chronic liver diseases, the use of mathematical equations combining tests may be effective as surrogate markers for the extent of fibrosis, steatosis, and inflammation as well as to predict the short-term prognosis in patients with liver disease.The use of laboratory tests is an important component of the assessment of patients with liver disease. With increasing frequency, elevations in the hepatic enzymes detected on routine screening tests provide the first evidence of liver disease in otherwise asymptomatic patients. In those patients with previously recognized hepatic dysfunction, laboratory tests can help establish the etiology of liver disease and provide valuable prognostic information. Rarely does a single test provide sufficient information to establish a diagnosis or assess severity of liver disease. A combination of tests such as serum bilirubin, albumin, aminotransferases, and alkaline phosphatase is sometimes referred to "liver function tests" or a "liver panel." These tests, in combination with the prothrombin time can provide an initial characterization of the etiology and severity of liver disease. However, one should recognize that the aminotransferases and alkaline phosphatase are tests that reflect hepatic injury not function. Traditionally liver diseases have been characterized as primarily hepatocellular or cholestatic based on the predominance of elevated aminotransferases or alkaline phosphatase. While an individual laboratory test may not provide a specific diagnosis, the pattern of abnormalities can suggest a general category of hepatic dysfunction. Although this distinction helps direct the initial evaluation there is often considerable overlap so that aggregating the diseases is less useful, as in the case...

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