GGT (gamma-glutamyl transferase) is an enzyme found in the liver, bile ducts, kidneys, pancreas, and other tissues. It plays a role in amino acid transport and glutathione metabolism. In routine blood testing, GGT is primarily used as a marker of liver and bile duct disease and as a sensitive indicator of alcohol consumption.

GGT is one of the most sensitive liver tests — it rises early in liver disease before other enzymes become abnormal, and it is particularly responsive to alcohol, even in moderate amounts. Elevated GGT alongside elevated ALP strongly suggests a hepatobiliary (liver and bile duct) source, helping to rule out a bone origin of the ALP elevation. GGT also rises with certain medications (particularly anticonvulsants and antiretrovirals), obesity, and metabolic syndrome.