Alanine Aminotransferase (ALT)

ALT (alanine transaminase) is an enzyme, a protein that speeds up certain chemical reactions in your body. It is found mainly in your liver, which contains far more ALT than other tissues do. Usually, you will have low levels of ALT in your blood. But when liver cells are damaged, they release ALT into the bloodstream. High levels of ALT in your blood may be a sign of a liver injury or disease. Some types of liver disease cause high ALT levels even before you have symptoms of the disease. So, an ALT blood test may help diagnose certain liver diseases early, when they may be easier to treat.

An ALT test is usually ordered as part of a group of liver function tests.

An ALT blood test is often done as part of a group of liver function tests. Liver function tests measure several different proteins, substances, and enzymes that show how well your liver is working. Your provider usually compares your ALT results with the results of the other liver tests to evaluate your liver health and decide if you need other tests to make a diagnosis.

Lower-than-usual levels of ALT in your blood are not common, and your provider may order more tests to investigate the cause.

Higher-than-usual ALT levels often mean liver cells are being damaged, which can happen with viral hepatitis, fatty liver disease, alcohol-related liver injury, certain medicines and supplements, or autoimmune liver disease. ALT often rises before symptoms appear.

Very high ALT (many times the upper limit of normal) is most often seen in acute hepatitis or other sudden liver injury. In hepatitis, ALT is usually as high as or higher than AST. Smaller increases may be seen in long-lasting (chronic) liver conditions.

If your results show you have a high level of ALT, it doesn't always mean that you have a medical condition that needs treatment. Many things can affect your results, such as your age, sex, weight, and certain medicines and dietary supplements. Your ALT levels can also be affected by intense exercise and where you are in your menstrual cycle.