VKORC1 and CYP2C9 Genotypes and Phenprocoumon Anticoagulation Status: Interaction Between both Genotypes Affects Dose Requirement

In a prospective follow‐up study of the effects of VKORC1 and CYP2C9 genotypes on the anticoagulation status of patients, we assessed the CYP2C9 and the VKORC1 C1173T genotypes of patients during the initial 6 months of phenprocoumon treatment. We used linear regression models and Cox proportional hazard models to determine the effects of the VKORC1 and CYP2C9 genotypes on phenprocoumon dose requirements, overanticoagulation, and time to achieve stability. Allele frequencies of interest within the cohort (N=281) were 40.8% VKORC1 T‐1173, 12.8% CYP2C9*2, and 6.9% CYP2C9*3. In patients with the VKORC1 CC genotype, carriers of a CYP2C9 polymorphism needed dosages that were nearly 30% lower than those for CYP2C9*1/*1 patients (P