Frequently Asked Questions about AST, ALT, GGTP, AP

Jeff Punch MD
Division of Transplantation
University of Michigan

What do these letters mean?

These letters are acronyms for enzymes – proteins inside of cells. AST for example stands for aspartate amino transferase. This enzyme used to be called serum glutamic oxalacetic transaminase (SGOT), hence the two names. ALT = amino alanine transferase, GGTP= gamma glutamyl transpeptidase, and AP= alkaline phosphatase. Different cells have different enzymes inside them, depending on the function of the cell. Liver cells happen to have lots of AST, ALT, and GGTP inside them. When cells die or are sick the enzymes leak out causing the blood level of these enzymes to rise, which is a way of determining if the cells in question are sick. ALT is more specific for liver disease than AST because AST is made in more places (e.g. heart, intestine, muscle). So the AST will rise after a heart attack or bruised kidney. GGTP and AP are said to be more specific for biliary disease since they are made in bile duct cells. In liver disease caused by excess alcohol ingestion, the AST tends to exceed the ALT, while the reverse is true to for viral hepatitis. However, this particular generalization is often wrong.

Some points:

•These tests have meaning, but they generally cannot be interpreted without clinical information. They are probably most useful to track, or follow a particular problem, but even then they often “bounce around” greatly.

•These numbers are not linear. An AST that is 300 is not twice as bad as 150 (normal is less than 50). We are used to numbers like temperature and dollars. If it is 94 degrees F outside, it is warmer than if it is 80 every time. And if one has 94 dollars, one has more money than if one has 80. Liver enzyme values don’t behave this way. An AST of 94 and 80 are essentially the same to a liver specialist.

•These numbers do not always detect all liver disease. Some very patients with severe advanced liver disease will have normal or nearly normal enzyme levels.

Are these numbers indicative of liver funtion?

Not really. Unfortunately, they are often called “liver function tests” or “LFT’s”, but in actuality, they do not measure function per se.

Then how is liver function measured?

Other tests including:
albumin and bilirubin, and prothrombin time are more truely measures of function, but clinical factors must be considered as well.