GPT and cholelithiasis: The most frequently used diagnosis and procedure codes (Table 3), however, were for investigation of various cholestatic and liver abnormalities, including, gamma-glutamyl transferase [25]; abnormal serum enzyme; other or unspecified [21]; jaundice [17]; elevation of aspartate aminotransferase (AST), alanine aminotransferase (ALT), or lactate dehydrogenase (LDH) [12]; obstruction of gallbladder or bile duct [11]; and cholelithiasis [10].