Serum Alkaline Phosphatase Gamma Glutamyl Transpeptidase The primary utility of these two enzymes is to serve as indicators of cholestasis. Cholestasis implies impairment of bile flow, which can be caused by physical or functional obstruction of the biliary tract within or outside of the liver. When severe enough, cholestasis will result in elevations of bilirubin in blood. The main value of these enzymes is their greater sensitivity for this abnormality as compared to serum bilirubin levels alone. Alkaline phosphatase, however, is less than entirely specific for this purpose, in that its activity in serum can be influenced by a variety of other factors. Gamma GT is more specific in general, and more sensitive in some instances.