A GGT blood test measures the amount of the enzyme gamma-glutamyl transpeptidase in the blood, according to MedlinePlus. The test is used to detect diseases of the liver or bile duct.
The GGT test is also done with other tests, such as ALT, ALP and bilirubin, to tell the difference between liver or bile duct disorders and bone disease. The test may also be done to screen for or monitor alcohol abuse, explains MedlinePlus. The normal GGT range is zero to 51 international units per liter. Drugs that can increase GGT levels include alcohol, phenytoin and phenobarbital. Birth control pills and the medication clofibrate can decrease GGT levels.