Alanine Aminotransferase (ALT)

What it measures

ALT is an enzyme inside liver cells. When liver cells are injured and their membranes are damaged, ALT leaks into the blood. So it mainly shows whether liver cells are being injured, not how well the liver is working.

What a high value may mean

The main cause is liver cell injury, which has many possible reasons. In addition, some drugs can injure the liver (such as anticonvulsants and steroids); severe muscle damage can raise ALT, for example aortic thromboembolism in cats; and hemolysis in a cat's blood sample can cause a false increase. The American Animal Hospital Association (AAHA) guidelines also note that hyperthyroid cats often have raised ALT or ALP.

What a low value may mean

Interpretation focuses on high values; clinical interpretation guides do not list a low ALT as something that needs action.

Usually read together with

ALP, bilirubin, and other liver values; muscle enzymes if a muscle source is suspected. In senior cats, thyroxine (T4) as well.

When to follow up

How much ALT is raised and why needs your vet's overall judgment, and they may suggest a recheck, more liver tests, or imaging. If the cat also stops eating, vomits, or shows jaundice (yellow gums or whites of the eyes), see the vet promptly.

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