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

A physician usually orders an ALT test (and several others) 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 ordered, 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, 
• individuals whose families have a history of liver disease, or 
• persons who take drugs that might occasionally damage the liver.

In persons with mild symptoms, such as fatigue or loss of energy, ALT may be tested to make sure they do not have chronic 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.

Very high levels of ALT (more than 10 times the highest normal level) are usually due to acute 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 come back 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 typically 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.

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

Many 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.