aspartate transaminase
Meaning of aspartate transaminase: A raised result announces that cells somewhere have been damaged without saying which cells, which is why it is never read alone.
Definition of aspartate transaminase
Aspartate transaminase is an enzyme that shuttles an amino group from one molecule to another, a routine step in the handling of amino acids. It sits inside cells of the liver, heart, skeletal muscle, kidney, brain and red blood cells, so only a trace circulates normally. When those cells are injured the enzyme leaks out and the blood level rises.
Where does it come from?
| Source | Situations that release it |
|---|---|
| Liver | Hepatitis of any cause, fatty liver disease, drug injury, bile duct obstruction |
| Skeletal muscle | Strenuous exercise, crush injury, muscle disease, seizures, injections into muscle |
| Heart | Damage to heart muscle, though troponin has long since replaced it for that purpose |
| Red blood cells | Breakdown of red cells, or a sample damaged during collection, which raises the figure with no illness behind it |
That last row is a practical trap. A difficult blood draw can produce a raised result on its own, which is one reason an unexpected value is repeated before anything is concluded from it.
Why is it read alongside ALT?
Alanine transaminase, the enzyme also called serum glutamate pyruvate transaminase, is concentrated in the liver and much scarcer elsewhere, so it is the more liver-specific of the pair. Read together they say considerably more than either alone. Both raised in similar proportion points to injury of liver cells generally. Aspartate transaminase disproportionately higher than alanine transaminase is a recognized pattern in alcohol-related liver injury, and also occurs when muscle rather than liver is the source. Very high values in both suggest sudden severe injury, such as a drug overdose or an interrupted blood supply, while a mild persistent rise suggests something chronic and smoldering.
What else moves the result?
Reference ranges differ between laboratories and between methods, so a value is read against the range printed on that report rather than a universal number, and it also varies with sex, body mass and muscle bulk. Two counterintuitive situations are worth knowing. Someone with advanced cirrhosis may have almost normal enzyme levels, because too few liver cells remain to leak any. And enzyme levels say nothing directly about how well the liver is working; that is judged by other measures such as clotting time, albumin and bilirubin. A laboratory test of this kind reports injury, not function.
Also known as
Used in a sentence
Laboratory testing showed an elevated aspartate transaminase level, prompting further evaluation of liver function.
Good to know
Because aspartate transaminase is present in heart and muscle tissue as well as the liver, a raised level is interpreted alongside more liver-specific markers before liver injury is assumed to be the cause.
Word origin
Where “aspartate transaminase” comes from
Formed from transaminase, an enzyme that transfers an amino group from one molecule to another, and aspartate, the amino acid it acts on.