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Liver function tests, also called hepatic panel, are blood tests used to measure liver function and damage. Abnormal test results do not always indicate liver disease. Some abnormalities are transient, they resolve without symptoms within a few weeks. Others may result from non-hepatic causes.
Two major functions of the liver are evaluated: the liver’s ability to produce blood plasma proteins, such as albumin and blood coagulation factors; and the liver’s ability to process bilirubin, a blood waste product.
Normal albumin level in the blood is between 40 and 60 grams per liter. A lower than normal albumin level may be a sign of liver disease. However, low serum albumin may also result from low protein diets, poor absorption of proteins, or abnormal loss of proteins in urine.
Because the liver synthesizes most of the factors required for blood clotting, reduced liver function leads to less coagulation factors produced and delayed coagulation. Prothrombin time, PT, or pro time test, measures the time the blood takes to clot. Normal PT is between 10.9 and 12.5 seconds. A higher than normal PT means the blood takes longer to clot, and may indicate liver damage. However, patients with bleeding disorders, vitamin K deficiency, and those who take blood-thinning medications, such as warfarin, may also have high PT without having liver problems.
Bilirubin is a waste product of red blood cell breakdown. Bilirubin is transported to the liver where it is processed and secreted into bile. Reduced liver function leads to accumulation of bilirubin in the blood, which gives the skin and the whites of the eyes a yellowish color known as jaundice. Plasma bilirubin level is increased in liver diseases and in some types of anemia.
When liver cells are injured, their content, including liver enzymes, are leaked into the bloodstream. The levels of these enzymes can be measured to assess the extent of liver damage. The higher their levels in the blood, the greater the extent of liver damage. However, because some of these enzymes are also present in other tissues, abnormal results may be due to causes outside the liver.
Four enzymes are usually included in a hepatic panel, two of which are involved in protein metabolism: aspartate transaminase, AST, and alanine transaminase, ALT. The other two - alkaline phosphatase, ALP, and gamma-glutamyltransferase, GGT, are components of bile ducts and involved in bile secretion. Between ALT and AST, ALT is more sensitive and specific for liver damage. Both AST and ALT normal values are higher in men than in women; their normal range is also higher in obesity. Between GGT and ALP, GGT is more specific for biliary disease. ALP is also present in bones and placenta, and its normal level increases during childhood growth, as well as pregnancy. ALP elevations may also be due to a number of non-hepatic illnesses including bone disease, chronic renal failure, congestive heart failure, cancers, and infection or inflammation.