Liver Function Tests

Excerpt

The liver, located in the right upper quadrant of the body and below the diaphragm, is responsible for several functions, including primary detoxification of various metabolites, synthesizing proteins, and producing digestive enzymes.The liver also plays a significant role in metabolism, regulation of red blood cells (RBCs), and glucose synthesis and storage. Typically when reviewing liver function tests, the discussion includes alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), 5'nucleotidase, total bilirubin, conjugated (direct) bilirubin, unconjugated (indirect)bilirubin, prothrombin time (PT), the international normalized ratio (INR), lactate dehydrogenase, total protein, globulins, and albumin. These tests can help determine the area of hepatic injury, and the elevation pattern can help organize a differential diagnosis.

The term "liver function tests "is a misnomer as many of the tests do not comment on the function of the liver but rather pinpoint the source of the damage. Elevations in ALT and AST in out of proportion to ALP, and bilirubin denotes a hepatocellular disease. An elevation in ALP and bilirubin in disproportion to ALT and AST would characterize a cholestatic pattern. A mixed injury pattern is defined as an elevation of alkaline phosphatase and AST/ALT levels. Isolated hyperbilirubinemia is defined as an elevation of bilirubin with normal alkaline phosphatase and AST/ALT levels.The R ratio has been used to assess whether the pattern of liver injury is hepatocellular, cholestatic, or mixed. The R ratio is calculated by the formula R =(ALT value÷ALT ULN)÷(alkaline phosphatase value÷alkaline phosphatase ULN). An R ratio of >5 is defined as hepatocellular, <2 is cholestatic, and 2–5 is a mixed pattern. The actual function of the liver can be graded based on its ability to produce albumin as well as vitamin K-dependent clotting factors.