Reading your liver function test: ALT, AST, GGT and the rest

The report arrives through a patient portal, in the post or across the GP's desk, and the liver section is a short column of abbreviations: ALT, AST, GGT, ALP, bilirubin, albumin. Each has a number, a unit and a reference interval beside it, and sometimes a flag or asterisk where a value falls outside that interval. Without a translation it reads like a code.

This guide explains what each line broadly reflects and why doctors tend to read the panel as a pattern. It will not tell you what your own numbers mean, which depends on your history, your medicines and the rest of your results. That is a conversation for your GP.

What the panel is

Healthdirect describes liver function tests, or LFTs, as common blood tests that measure proteins, enzymes and other substances in the blood. They are the third most common pathology test ordered by Australian GPs, according to a teaching guide from General Practice Supervision Australia. You need a request form from your doctor and usually have the blood taken at a pathology collection centre. Healthdirect says no special preparation is needed, although other tests ordered on the same form may require some.

The name is slightly misleading. The same teaching guide calls it a misnomer, because most of the panel measures whether liver cells or bile ducts are under strain. Only bilirubin and albumin reflect the work the liver actually does.

The six lines, one by one

• ALT (alanine aminotransferase). An enzyme found mostly in the liver, with smaller amounts in the kidneys, heart and muscles, according to Pathology Tests Explained. It is released into the blood when liver cells are damaged.
• AST (aspartate aminotransferase). A similar enzyme, but one that also comes from skeletal and heart muscle. For that reason the GP teaching guide describes it as less specific to the liver than ALT.
• GGT (gamma-glutamyl transferase). An enzyme from the bile-facing surface of liver cells. Healthdirect says it can indicate liver stress. It is associated with alcohol, though loosely, as explained below.
• ALP (alkaline phosphatase). Linked to bile flow, but also made in bone, the intestine and the placenta. Children have higher levels because their bones are growing, and levels rise naturally in pregnancy.
• Bilirubin. A substance produced when old red blood cells break down, which the liver removes from the body. Healthdirect describes it as reflecting bile production and excretion.
• Albumin. The most common protein in the blood, made by the liver. It helps regulate fluid in the body.

Why doctors read a pattern

Healthdirect is clear that these results do not diagnose a specific condition. They can show signs of liver disease or damage, and some of them can be affected by conditions elsewhere in the body.

A primer for GPs, written by chemical pathologist Penelope Coates for the Royal Australian College of General Practitioners journal Australian Family Physician, explains the usual approach. Doctors look at which lines have moved together. Raised ALT and AST suggest that liver cells themselves are affected. Raised ALP with raised GGT commonly reflects a problem with bile flow, or the effect of alcohol or a medicine. ALP raised on its own is typically traced to bone, and a raised GGT alongside it is what suggests the liver as the source.

Albumin shows how context changes the reading. Coates writes that low albumin can indicate severe liver disease but more often has other causes, among them inflammation, poor nutrition and pregnancy.

Why a mildly raised result is often rechecked

A reference interval is built to cover most healthy people, not all of them. Coates points out that, by definition, 2.5% of the healthy population may have an abnormal result at any one time, and that such results are usually mild. She notes that an isolated, unexpected minor abnormality may simply be repeated after a short interval, such as a week, and cites research in which almost a third of results returned to the normal range on repeat testing. A result that persists is more likely to matter.

This article deliberately gives no numbers for what is normal. The interval printed beside each result on your own report is the one your GP will work from. Coates adds that using the same laboratory for repeat tests is preferable, because methods differ for some measurements.

Things that can shift a result

Hard exercise. Because AST and, to a lesser degree, ALT are also found in muscle, strenuous exercise can raise them. In a study published in the British Journal of Clinical Pharmacology, fifteen healthy men who were not used to weightlifting completed a single one-hour weights session. Their AST and ALT rose significantly and stayed raised for at least seven days. It was a small study in men only, but it is a reason to mention recent heavy training to your GP.

Alcohol. GGT is the line most associated with drinking, yet it is an imperfect guide. Coates writes that only about 70% of isolated GGT rises are due to excess alcohol, that 30% of people who drink heavily have a normal GGT, and that levels stay elevated for two to three weeks after heavy drinking stops.

Medicines. Medication appears on Coates's list of common causes of mildly raised ALT and AST, alongside infection, alcohol and fatty liver. The GP teaching guide calls a careful medication history vital, including anything bought over the counter. Do not stop a prescribed medicine because of a pathology result without speaking to your GP first.

Illness and fasting. Up to 5% of the population has Gilbert syndrome, which Coates describes as a common and benign trait in which bilirubin runs a little high and rises further during acute illness or fasting.

What a normal result does not tell you

The GP teaching guide makes a point that is easy to miss: the size of an abnormal result is not a good indicator of how severe a liver condition is, and normal results do not necessarily mean the absence of liver disease. Coates gives the example of cirrhosis, where enzyme levels may drop back within the reference range. This is one reason GPs weigh a person's history and risk factors alongside the numbers.

If results are abnormal, Healthdirect says a doctor may suggest further tests, such as an ultrasound to check the liver's shape and size, a CT or MRI scan, or more blood tests to find the cause.

What to ask your GP

• Which of my results are outside the laboratory's reference interval, and by how much?
• Do the results form a pattern, and what does that pattern suggest to you?
• Could anything I have done or taken recently have affected them, such as heavy exercise, alcohol, an illness or a medicine?
• Should the test be repeated, and when?
• Are any further tests needed, and what would they be looking for?

Sources

1. Healthdirect Australia, Liver function tests (last reviewed March 2025).
2. Pathology Tests Explained, Liver function tests (LFT).
3. Pathology Tests Explained, Alanine aminotransferase (ALT).
4. Pathology Tests Explained, Alkaline phosphatase (ALP).
5. Coates P, Australian Family Physician (Royal Australian College of General Practitioners), Liver function tests (2011).
6. General Practice Supervision Australia, Abnormal liver function tests (teaching aid) (2023).
7. Pettersson J and colleagues, British Journal of Clinical Pharmacology, Muscular exercise can cause highly pathological liver function tests in healthy men.