Journal of the Medical Association of Thailand Vol 92, No 12:DECEMBER 2009 0125-2208 92 12 2009 Dec Prevalence of CYP2C9 and VKORC1 Mutation in Patients with Valvular Heart Disease in Northern Thailand 1597 EN Srun Kuanprasert Sakchai Dettrairat Poonsub Palacajornsuk Warunee Kunachiwa Arintaya Phrommintikul Original Article Patients with valvular heart disease who regularly took a steady maintenancewarfarin dose for at least one month were recruited into the present study. Patients who had taken amiodaroneor an anti-inflammatory drug were excluded. Clinical data were obtained from medical records. Five millilitersof whole blood was drawn from each patient for gene analysis and prothrombin time with internationalnormalized ratio (INR) measurement. From 242 patients, CYP2C9 *1/*1 was found in 230 patients (95%) and CYP2C9 *1/*3 was foundin 12 patients (5%). Neither mutant CYP2C9*2 allele nor individuals homozygous for CYP2C9*3 wereobserved. Regarding VKORC1, haplotype AB was found in 83 patients (34.3%) and haplotype AA was foundin 154 patients (63.6%). Haplotype BB (wild type) was found in five patients (2.1%). The prevalence of CYP2C9 *1/*1 is high while the prevalence of CYP2C9*2 and CYP2C9*3 isvery low. VKORC1 haplotype AA is the most common among the Northern Thai population. Further studyregarding pharmacogenetic and non-genetic factors to develop warfarin-dosing algorithm is warranted. Pharmacogenomics Polymorphisms Warfarin Anticoagulant