• For warfarin phenotyping, blood samples will be obtained at 0, 3, 6, 12, 24, 36, 48, 72 and 96 hours following warfarin plus vitamin K administration to determine CYP2C9 activity (S-warfarin CL/F, mL/min). For the first 12 hours, blood will be obtained through an intravenous (IV) catheter. Thereafter it will be via venipuncture.