What do ALT, AST and GGT on my health check mean?

ALT, AST and GGT are enzymes, proteins that speed up chemical reactions in the body, and on a health check they sit in the liver section next to ALP, bilirubin and albumin. MedlinePlus, from the US National Library of Medicine, explains that ALT, AST, GGT and ALP are made mainly in the liver, that each test looks at something slightly different, and that a doctor compares all of them together because on their own they usually cannot tell the exact cause of a result. The line between "normal" and "high" also differs by lab and by sex: a 2017 US guideline for doctors gives a healthy ALT of 29 to 33 U/L for men and 19 to 25 U/L for women.

What is in the liver section of a health check report?

MedlinePlus calls this group of tests liver function tests, also called a liver panel. The values in it, and the names printed, vary between hospitals, but most reports include the ones below.

The usual values and their other names

Most of the names and descriptions come from the MedlinePlus pages for each test. The units printed on reports are often U/L for the enzymes, mg/dL for bilirubin and g/dL for albumin, but check your own report.

Why they are read as a group

MedlinePlus explains that when any liver result is outside the range, the provider compares the results of all the substances measured and looks for patterns. The provider also considers your symptoms, history and the medicines you take, and may order other tests. That is why reading one number on its own, or comparing it with a friend's, tells you little.

What does each value look at?

Each value answers a slightly different question. The descriptions below follow MedlinePlus.

ALT (SGPT)

The MedlinePlus ALT page explains that ALT is found mainly in the liver and that usually only low levels are in the blood. When liver cells are damaged, they release ALT into the blood. Because ALT is mostly in the liver, it is the more liver-specific of the two transaminases. MedlinePlus adds that a high ALT does not always mean a health problem, and that age, sex, weight, certain medicines and dietary supplements, intense exercise and where you are in your menstrual cycle can affect it.

AST (SGOT)

The MedlinePlus AST page explains that AST is also found mainly in the liver, but also in the heart, muscles and other tissues. When cells that contain AST are damaged, it is released into the blood. MedlinePlus lists pregnancy, exercise, certain medicines, age and sex among the things that can affect results, and alcohol among the things that can raise AST.

GGT (Gamma-GT)

The MedlinePlus GGT page explains that GGT is found throughout the body but mainly in the liver, and that it may leak into the blood when the liver or bile ducts, the tubes that carry bile, are damaged. It says a GGT test on its own cannot tell the specific cause. GGT is usually read together with ALP and the other liver values. MedlinePlus also says that drinking alcohol increases GGT.

ALP (alkaline phosphatase)

The MedlinePlus ALP page explains that ALP is found in all body tissues, with higher amounts in the liver, bile ducts and bones, and that each part of the body makes a different type. A standard ALP test cannot tell which type is high, so it is read together with other tests. MedlinePlus notes that children and teens may have high ALP because their bones are growing, that pregnancy can raise it, and that a fatty meal before the test may cause a small increase.

Bilirubin

The MedlinePlus bilirubin page explains that bilirubin is a yellowish substance made during the body's normal breakdown of old red blood cells, and that a healthy liver removes most of it through the bile ducts. Normal ranges vary from lab to lab and by sex. MedlinePlus lists some medicines, certain foods and strenuous exercise as possible causes of higher-than-normal bilirubin.

Albumin and total protein

The MedlinePlus albumin page explains that albumin is a protein made by the liver that keeps fluid inside blood vessels and carries hormones, vitamins and enzymes around the body. Albumin can be lower than normal if you have not eaten for 24 to 48 hours, during pregnancy and with certain medicines, and higher with dehydration. Total protein measures albumin plus other proteins called globulins.

Why are the ranges different for men and women?

Studies and guidelines draw the upper line for ALT higher for men than for women, and where the "high" line is drawn has changed over time.

What the research found

A 2002 study by Prati and colleagues looked at 6,835 first-time blood donors at a university hospital in Milan, Italy. From the donors least likely to have any liver problem, the authors proposed upper limits of 30 U/L for men and 19 U/L for women, lower than the 40 U/L and 30 U/L used at the time. The 2017 US guideline by Kwo and colleagues gives a healthy ALT of 29 to 33 U/L for men and 19 to 25 U/L for women.

Where different sources draw the line

The Thai army study comes from Sakboonyarat and colleagues at Phramongkutklao College of Medicine, who analysed 244,281 health check records of personnel aged 35 to 60. These are examples, not your lab's range. Your report prints the range your lab uses, and the doctor reads your result against it.

Age matters too

MedlinePlus lists age and sex among the things that affect ALT and AST, and notes that bilirubin ranges differ for infants and children, and that growing children and teens can have high ALP. A value that is normal for one person may be read differently for another.

What can change these values besides the liver itself?

Many everyday things can move liver values. MedlinePlus lists the following for each test.

Factors listed by MedlinePlus

Fasting and GGT

A 2026 study by Shin and colleagues, laboratory medicine researchers in Korea, looked at 9.76 million test results from 101,148 adult outpatients at one university hospital. GGT was about 14.8% higher when people had fasted less than 8 hours, compared with longer fasting. Between fasting 8 to 12 hours and fasting more than 12 hours, no test it examined differed by more than the accepted margin. The data come from one hospital and were collected for routine care, so they show a pattern rather than a rule for every person. MedlinePlus, on the other hand, says GGT levels go down after meals, so the sources do not yet agree. Follow the clinic's instructions on fasting.

What does GGT have to do with alcohol?

MedlinePlus says drinking alcohol increases GGT, and GGT is one of the values doctors look at when they ask about drinking.

A study in southern Thailand

A study by Assanangkornchai and colleagues at Prince of Songkla University, published in 2003, interviewed 1,005 adults aged 35 and over in two urban and five rural areas of southern Thailand, using a standard drinking questionnaire (AUDIT). Blood was taken from 200 randomly selected people to measure GGT.

What this does and does not show

GGT was above normal more often among people who drank more, but 15% of non-drinkers also had a high GGT, and about half of the higher-risk drinkers did not. Only 200 people in one region had blood taken, and the data are more than 20 years old. GGT on its own cannot tell a doctor how much someone drinks, which is why being honest about your drinking helps the doctor read it.

What should I ask the doctor about my liver values?

The doctor reads your liver values together, against your lab's ranges and your history.

What to bring and tell

• this report and any earlier reports
• a list of every medicine, vitamin, supplement and herbal product you take
• how much you usually drink and whether you smoke
• any hard exercise, illness or unusual eating in the week or weeks before the test

What to ask

• which reference range your report uses, and why a value was flagged
• what the pattern of your liver values means together
• whether and when to test again, and how to prepare

If one or more values were flagged and you want to know what to do next, see Liver numbers a little high at my health check: what first? For preparing for a health check, see My health check is coming up again. What should I change?

Do not start, stop or change regular medicines or supplements on your own because of a result. Ask the doctor or pharmacist first.