Biomarker 101: Alanine aminotransferase (ALT)

ALT is the liver enzyme most specific to the liver itself — which is what makes it useful, and also what makes it move for entirely ordinary reasons.

What ALT is

Alanine aminotransferase is an enzyme involved in amino acid metabolism, concentrated in liver cells. When those cells are stressed or damaged, ALT leaks into the bloodstream, so the level in your blood reflects what is happening in the liver.

It is usually reported alongside AST, ALP and GGT as part of a liver panel. ALT is the most liver-specific of the group, which is why it carries weight when the others are ambiguous.

Why it's measured

Liver problems are frequently silent in their early stages, which is the main argument for measuring rather than waiting for symptoms. Where symptoms do appear they can include appetite loss, nausea, abdominal discomfort or swelling, itching, easy bruising, or yellowing of the skin.

ALT forms part of a general health panel and is used to investigate or monitor known liver conditions. Importantly, a normal ALT does not rule liver disease out — some conditions progress with normal enzymes.

What the numbers mean

Reference ranges differ by laboratory and by sex, with women's ranges typically lower than men's. Australian and New Zealand laboratories report ALT in U/L, and your result should be read against the range printed on your own report rather than a general figure.

What moves ALT

Alcohol is a common contributor, and liver effects can occur below the level people assume is safe. National alcohol guidelines in both Australia and New Zealand are worth knowing.

Fatty liver associated with excess body fat, particularly around the abdomen, is now the most common cause of raised ALT in comparable populations.

Medications and supplements can raise it — prescription, over-the-counter, herbal and sports supplements alike. This is worth mentioning to your doctor rather than acting on yourself.

Strenuous exercise in the days before a test can lift ALT without anything being wrong.

Less commonly, viral or autoimmune hepatitis, iron overload conditions, coeliac disease and several inherited disorders raise ALT.

Low ALT is generally not clinically significant on its own.

What ALT won't tell you

• It doesn't identify a cause. ALT indicates liver cells are under stress, not why.
• Normal doesn't mean clear. Some liver conditions progress with normal enzyme levels.
• One raised result isn't a diagnosis. Repeat testing after a period without alcohol and without heavy exercise is common before anything is concluded.
• It rarely indicates cancer on its own. Liver cancer generally arises against a background of long-standing liver damage rather than showing first as an isolated raised ALT.

The bottom line

ALT is the most liver-specific enzyme on a standard panel, and one of the easiest to nudge upward through ordinary things — a heavy weekend, a hard training week, a new supplement. Read alongside the rest of the liver panel and repeated when necessary, it is genuinely informative.