increases the levels of this enzyme are as sensitive as alkaline phosphatase for detecting cholestasis and biliary obstruction but are more specific, almost always indicating hepatobiliary dysfunction. 

because levels of alkaline phosphotase and 5'-nucleotidase do not always correlate, one can be normal while the other is increased. 

PT or INR is a valulable measure of the livers ability to synthesize fibrinogen and vitamin K--dependent clotting factors:

factors II (prothombrin), V, VII, and X. Changes occur rapidly because some of the involved clotting factors have shtort biologic half lives. (6 h factor for VII). Abnormalities indicate severe hepatocellular dysfunction, and ominous sign of acute liver disorders. 

levels do not increase because of bone lesions, during childhood, or during pregnancy. 

Howevever, alcohol and certain drugs (anticonvulsants and warfarin)  and introduce hepatic microsomal (cyto. P450) enzymes, markedly increasing GGT and thus somewhat limiting its specificity.