Gamma-glutamyl transferase (GGT) is a protein (enzyme) that speeds up chemical reactions in the body. It is found mainly in the liver and the bile ducts.
A blood test to measure GGT levels is done to check for liver damage, especially damage caused by heavy alcohol use. A GGT test is usually done along with other tests (such as alanine aminotransferase, or ALT) because it can't usually tell what disease is causing the liver damage.
Gamma-glutamyl transferase is also called gamma-glutamyl transpeptidase (GGTP).