Alanine aminotransferase (ALT) is an enzyme found mostly in the cells of the liver and kidney. Much smaller amounts of it are also found in the heart and muscles. In healthy individuals, ALT levels in the blood are low. When the liver is damaged, ALT is released into the bloodstream, usually before more obvious signs of liver damage occur. This makes ALT a useful test for detecting liver damage.  Sample   Blood (serum)  Reference   Female, less than 34 U/l; male, less than 45 U/l  Frequency   Daily