The gamma-glutamyl transpeptidase test is a blood test that measures the levels of gamma-glutamyl transpeptidase in the blood, according to the University of Rochester Medical Center. This test helps determine any damage in the liver or its bile ducts, differentiate between liver and bone disease, and verify chronic alcohol abuse.
GGT enzymes are commonly found in the liver, kidney, pancreas, heart and brain, states MedlinePlus. Its normal value ranges between 8 to 38 U/L for men and women older than 45 years old, and between 5 to 27 U/L for women younger than 45 years old, as reported by the University of Rochester Medical Center. MedlinePlus advises patients that certain medications, such as phenytoin, phenobarbital, birth control pills and clofibrate, can affect the result of the test.