ALT (Alanine Aminotransferase) alanine transaminase, is an enzyme found mostly in the liver. When liver cells are damaged, they release ALT into the bloodstream. An ALT test measures the amount of ALT in the blood. High levels of ALT in the blood can indicate a liver problem, even before you have signs of liver disease, such as jaundice, a condition that causes your skin and eyes to turn yellow. An ALT blood test may be helpful in early detection of liver disease. An ALT blood test is a type of liver function test. Liver function tests may be part of a regular checkup. The test can also help diagnose liver problems.