The ALT test detects liver injury. ALT values are usually assessed along with the values found for other enzymes, such as alkaline phosphatase (ALP) and aspartate aminotransferase (AST), to help determine which form of liver disease is present.

A doctor usually requests an ALT test with other laboratory investigations to evaluate a patient who has symptoms of a liver disorder. Some of these symptoms include jaundice, dark urine, nausea, vomiting, abdominal swelling, unusual weight gain, and abdominal pain. ALT can also be used, either by itself or with other tests, for:

• persons who have a history of known or possible exposure to hepatitis viruses,
• those who drink too much alcohol,
• those whose family have a history of liver disease,
• people who take drugs that might damage the liver.

In people with mild symptoms, such as tiredness or loss of energy, ALT may be tested to make sure they do not have chronic (long-term) liver disease. ALT is often used to monitor the treatment of persons who have liver disease, to see if the treatment is working, and may be ordered either by itself or along with other tests.

NOTE: A standard reference range is not available for this test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test. Lab Tests Online strongly recommends that you discuss your test results with your doctor. For more information on reference ranges, please read Reference Ranges and What They Mean.

Very high levels of ALT (more than 10 times the highest normal level) are usually due to acute (short-term) hepatitis, often due to a virus infection. In acute hepatitis, ALT levels usually stay high for about 1–2 months, but can take as long as 3–6 months to return to normal. 

ALT levels are usually not as high in chronic hepatitis, often less than 4 times the highest normal level: in this case, ALT levels often vary between normal and slightly increased, so doctors will order the test frequently to see if there is a pattern. In some liver diseases, especially when the bile ducts are blocked, when a person has cirrhosis, and when other types of liver cancer are present, ALT may be close to normal levels.

An injection of medicine into the muscle tissue, or strenuous exercise, may increase ALT levels.

Certain drugs may raise ALT levels by causing liver damage in a very small percentage of patients taking the drug. This is true of both prescription drugs and some 'natural' health products. If your doctor finds that you have a high ALT, tell him or her about all the drugs and health products you are taking.