ALT (alanine aminotransferase) is an enzyme found predominantly in liver cells (hepatocytes), with smaller amounts in the kidneys, heart, and skeletal muscle. Its primary role is in amino acid metabolism. Because it is so highly concentrated in liver cells, ALT is the most liver-specific enzyme in routine blood testing — when liver cells are damaged or inflamed, ALT leaks out of cells and into the bloodstream, causing blood levels to rise in direct proportion to the degree of liver injury.

ALT is a cornerstone of liver function testing and is included in virtually every standard blood panel. It is the preferred marker for detecting liver cell damage (hepatocellular injury) — conditions including non-alcoholic fatty liver disease (NAFLD), viral hepatitis, alcohol-related liver disease, drug-induced liver injury, and autoimmune hepatitis all characteristically raise ALT. Because ALT is more liver-specific than AST, it is particularly useful for distinguishing liver injury from muscle or cardiac injury when used alongside AST.