Overview of ALT and AST Liver Enzymes
By Charles Daniel
Medically reviewed by Richard N. Fogoros, MD
Updated November 08, 2018
Highlights
“Any elevation in an enzyme level may be a sign of a liver problem, and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are two of the enzymes central to such an investigation.
When used comparatively, AST and ALT can help identify liver toxicity, liver disease, or liver damage.
Roles of AST and ALT
Aspartate aminotransferase (AST) is found in a variety of tissues, including the liver, brain, pancreas, heart, kidneys, lungs, and skeletal muscles. If any of these tissues are damaged, AST will be released into the bloodstream. While increased
AST levels are indicative of a tissue injury, it is not specific to the liver per se.
By contrast, alanine aminotransferase (ALT) is found primarily in the liver.
——Any elevation of the ALT is a direct indication of a liver injury, whether minor or severe. ——
Sustained increases are more problematic as they suggest an underlying disease and a greater likelihood of liver damage.
Normal Lab Values
AST and ALT are measured in international units per liter (IU/L). The normal levels vary based on a person’s body mass index (BMI) as well as the individual lab’s reference value.
Generally speaking, the normal reference value for adults is:
AST: 8 to 48 IU/L
ALT: 7 to 55 IU/L
The high end of the reference range is referred to as the upper limit of normal (ULN).
Mild elevations are generally considered to be two to three times the ULN. With some liver diseases, the level can exceed 50 times the ULN. Levels this high are described as deranged.
AST/ALT Ratio
While it may seem that a high ALT is all that is needed to diagnose liver disease, its relationship to AST can provide valuable clues as to what exactly is going on and whether the issue is acute (occurring suddenly and progressing rapidly) or chronic (long-standing or persistent).
If the liver sustains an acute injury, you can expect to see a sudden spike in the ALT.
On the other hand, if a liver disease is slowly progressing, the damage incurred by the liver will gradually affect other organs as well. As these organs are damaged, the AST will begin to rise.
This enzyme relationship can be described diagnostically with the AST/ALT ratio. This is a calculation that compares the levels of AST and ALT in your blood. Depending on which value is elevated and the extent of that elevation, doctors can often get a pretty strong indication as to what disease is involved.
What the AST/ALT Ratio Reveals
An AST/ALT ratio higher than one (where the AST is higher than ALT) is suggestive of cirrhosis.
An AST/ALT ratio higher than 2:1 (where the AST is more than twice as high as the ALT) is suggestive of alcoholic liver disease.
However, a disease cannot be diagnosed by the pattern of elevation alone. The magnitude of elevation described in multiples of the ULN also needs to be evaluated.
It is only when the magnitude is above a certain threshold that the ratio can be considered diagnostic.
Among the examples:
With alcoholic fatty liver disease, the AST would generally be more than eight times the ULN and the ALT more than five times the ULN.
With non-alcoholic fatty liver disease, the AST and ALT would both be more than four times the ULN.
With ischemic hepatopathy (also known as shock liver), the AST and ALT would be over 50 times the ULN.
A diagnosis would also be supported by other liver enzyme test results, including albumin, bilirubin, and gamma-glutamyl transpeptidase (GGT).”