Liver Function Tests
The term liver function tests is somewhat misleading because most such tests detect inflammation of or damage to the liver, not the liver's metabolic or bile-secreting functions (see Liver : Functions of the Liver). Such inflammation or damage can be present before the liver's actual functioning is affected. Liver function tests are blood tests that represent a noninvasive way to screen for the presence of liver disease (for example, hepatitis in donated blood) and to measure the severity and progress of liver disease and its response to treatment.
Laboratory tests are generally effective for the following:
Detecting liver inflammation, damage, or dysfunction
Assessing the severity of liver injury
Monitoring the course of liver diseases and a person's response to treatment
Refining the diagnosis
Liver function tests are done on blood samples and measure the levels of enzymes and other substances produced by the liver. These substances include
Levels of some of these substances measure how well the liver performs its normal functions of making proteins and secreting bile. Levels of other substances detect the presence and degree of liver inflammation. What constitutes a normal value for many of these tests can be found in see Table: Blood Tests*. However, sometimes values can be much higher than normal, usually because a person has another disorder.
One test of liver function is the prothrombin time (PT), which is used to calculate the international normalized ratio (INR). Both the PT and the INR are measures of the time needed for blood to clot (the liver synthesizes some proteins necessary for blood clotting, called blood clotting factors). An abnormal PT or INR result can indicate an acute liver disorder. In chronic liver disorders, an increasing PT or INR typically indicates progression to liver failure.