Alkaline phoshpatase is an enzyme found in the liver and bone. Blood levels of the enzyme are elevated in some types of liver disease. Children, especially teenagers, normally have higher blood levels of ALP than adults. This is related to the rapid growth and development of their bones. Compared to the transaminases like ALT, alkaline phosphatase tends to be higher in diseases associated with injury to the bile-secreting part of the liver's activity.

The alkaline phosphatase and the GGT are elevated in a large number of disorders that affect the drainage of bile, such as gallstone or tumor blocking the common bile duct, or alcoholic liver disease or drug-induced hepatitis, blocking the flow of bile in smaller bile channels within the liver. The alkaline phosphatase is also found in other organs, such as bone, placenta and intestine. For this reason, the GGT is utilized as a supplementary test to be sure that the elevation of alkaline phosphatase is indeed coming from the liver or the biliary tract. In contrast to the alkaline phosphatase, the GGT is not elevated in diseases of the bone, placenta or intestine. Mild or moderate elevation of GGT in the presence of a normal alkaline phosphatase is difficult to interpret and is often caused by changes in the liver cell enzymes induced by alcohol or medications, but without causing injury to the liver.