The biochemical hub of the body, the liver, has a variety of transaminases to synthesize and break down amino acids and to interconvert energy storage molecules. The concentrations of these in the serum (the non-cellular portion of blood) are normally low. However, if the liver is damaged, the hepatocyte cell membrane becomes more permeable and some of the enzymes leak out into the blood stream. The two transaminases commonly measured are alanine transaminase (ALT) and aspartate transaminase (AST). These levels previously were called the serum glutamate-pyruvate transaminase (SGPT) and the serum glutamate-oxaloacetate transaminase (SGOT). Elevated levels are quite sensitive for liver injury, meaning that they are likely to be present if there is injury. However, they may also be elevated in other conditions. ALT is not commonly found outside the liver; AST too is most commonly found in the liver, but also in significant amounts in cardiac (heart) and skeletal muscle. In fact, measurement of these used to be part of diagnosing heart attacks, although newer enzymes and proteins that are more specific for cardiac damage have largely replaced this usage.
In general, any damage to the liver will cause medium elevations in these transaminases (usually called liver enzymes, though of course they are not the only enzymes in the liver). And diagnosis requires synthesis of many pieces of information, including the patient's history, physical examination, and possibly imaging or other laboratory examinations. However, very high elevations of the transaminases suggests severe liver damage, such as viral hepatitis, liver injury from lack of blood flow, or injury from drugs or toxins. Most disease processes cause ALT to rise higher than AST; AST levels double or triple that of ALT are consistent with alcoholic liver disease.