(USMLE topics) Liver labs: AST, ALT, bilirubin, albumin, prothrombin time, ALP, and GGT.

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Liver function tests, also called hepatic panel, are blood tests used to monitor liver function and damage. These tests provide insights into several aspects of liver health, notably:
- the liver’s ability to synthesize enzymes and proteins,
- the liver’s ability to process bilirubin and secrete bile,
- and the extent of liver damage.
Abnormal liver function test results do not always indicate liver disease. Some abnormalities are transient; or they may result from different, non-hepatic causes.
The synthetic function of the liver can be assessed based on its ability to produce plasma proteins, such as albumin and coagulation factors. Serum albumin levels FALL with all liver diseases. However, low serum albumin may also result from low protein intake, malabsorption or abnormal loss of proteins in urine.
Prothrombin time, PT, or pro time test, measures the time the blood takes to clot. A decline in liver function leads to less coagulation factors produced and delays coagulation time. However, a high PT may also indicate a bleeding disorder, vitamin K deficiency, or use of blood-thinning medications, such as warfarin.
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. Two enzymes are usually included in a hepatic panel: aspartate transaminase, AST, and alanine transaminase, ALT; both are involved in protein metabolism in the liver. AST and ALT are present at high concentrations in the liver, but they are also found in a number of other tissues. ALT is more sensitive and specific for liver damage than AST. AST and ALT elevations may also result from non-hepatic causes.
Bilirubin is a water-insoluble product of normal heme breakdown. Bilirubin is transported to the liver loosely bound to albumin. The liver converts bilirubin into a water-soluble form to be secreted into bile. Accumulation of bilirubin in the blood indicates problems with biliary function. High levels of bilirubin give the skin and the whites of the eyes a yellowish color known as jaundice.
Besides bilirubin, two enzymes are measured for biliary function: alkaline phosphatase, ALP, and gamma-glutamyltransferase, GGT. Both enzymes are found in the tiny bile ducts, called canaliculi, of the liver; but they are also present in several other tissues. Damage to the biliary tract releases these enzymes into the bloodstream. Elevated GGT is more specific for biliary disease compared to ALP. ALP elevations may also be due to a number of non-hepatic causes.

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