 To date, only one prospective study has been published on warfarin dosing based upon cytochrome P450 2C9 genotyping. It was a negative study that failed to prove the hypothesis that genotyping would lead to better control of warfarin dosing. 48 orthopedic surgery patients were studied with genotyping and screening for nongenetic factors that might affect warfarin dosing and bleeding risk. The warfarin dosing algorithms based upon the genetic screening led to achievement of a stable and therapeutic INR in both the wild type group and the CYP2C9 variant groups with a similar time course. Nevertheless, despite pharmacogenetics-based dosing, patients with variants were at higher risk for excessive anticoagulation, defined as an INR greater than 4.0. 