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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.
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