ALT is an enzyme found predominantly in liver but with a moderate-sized component in kidney and small quantities in heart and skeletal muscle. In general, most ALT elevations are due to liver disease, although large amounts of tissue damage in the other organs mentioned may also affect serum levels. In fact, severe myositis or rhabdomyolysis can sometimes raise ALT to levels ordinarily associated with acute hepatitis virus hepatitis. ALT levels are elevated to approximately the same degree and frequency as AST in hepatitis virus hepatitis, infectious mononucleosis, and drug-induced acute liver cell injury. ALT levels are elevated less frequently than AST and usually to a lesser degree in acute alcoholic liver disease or active cirrhosis, liver passive congestion, long-standing extrahepatic bile duct obstruction, and metastatic tumor to the liver. ALT has been used predominantly to help confirm liver origin of an AST increase (although there are some limitations in ALT specificity) and occasionally as an aid in the differential diagnosis of liver disease by means of the AST/ALT ratio.

One report found that the normal ALT mean value was about 1.5 times higher in African-American males and 1.8 times higher in Hispanic males than in European males. The same study found that European male mean values were 40% higher than those of European females, African-American females were 20% higher, and Hispanic females, 40% higher.