Presence of jaundice, pruritus and clay coloured stools indicates cholestasis. Elevated levels of alkaline phosphatase, serum GGT and 5' nucleotidase indicates cholestasis. 5' nucleotidase is an enzyme specific to the liver. It is elevated in patients with liver diseases, especially those associated with cholestasis. Alkaline phosphatase is found primarily in the liver and bones. In the liver it is expressed by the bile duct epithelium and its levels are increased in conditions of biliary obstruction. Gamma glutamyl transpeptidase enzyme is found in the hepatocytes and are released from the bile duct epithelium. Elevated levels of this enzyme is an early marker and also a sensitive test for hepatobiliary disease. Ref: Schwaz's Principles of Surgery, 9e chapter 31. Primary Care: The A and Science of Advanced Practice Nursing By Lynne M Dunphy, page 505.