It's a test that measures the amount of the enzyme GGT in the blood. It's used to detect diseases of the liver, bile ducts, and kidney and to differentiate liver or bile duct disorders from bone disease.

Greater-than-normal levels may indicate:

congestive heart failure
cholestasis (congestion of the bile ducts)
cirrhosis
hepatic (liver) ischemia (blood deficiency)
hepatic (liver) necrosis (tissue death)
hepatic tumor
hepatitis
hepatotoxic drugs

Drugs that can increase GGT levels include alcohol, phenytoin, and phenobarbital. So if you drink a lot, it could skew the results.