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Objective: To identify the effect of CYP2C9 and VKORC1 genetic variants on warfarin dosage in the Thai population with DVT.
Material and Method: Genotyping of CYP2C9 (*2 and *3) and VKORC1 promoter (-1639G>A) variants were carried out in 97 Thai DVT patients receiving constant warfarin therapy and with a stable international normalized ratio using real-time PCR assays.
Results: VKORC1 AA, GA, and GG genotype frequencies were found to be 49.5%, 46.4%, and 4.1%, respectively, while those of CYP2C9 genotypes were 88.7% for *1/*1 and 11.3% for *1/*3. The CYP2C9*2 variant was not present in the patients studied. The mean daily warfarin dose required to maintain a therapeutic INR differed significantly according to VKORC1 genotype, with 3.6 mg/day required for AA, 4.7 mg/day for GA, and 7.4 mg/day for GG (p-value <0.001). The CYP2C9 genotype did not significantly affect the warfarin dosage requirement (p-value = 0.29).
Conclusion: These findings underline the impact of VKORC1 genotypes on the wide variation in warfarin maintenance dosing in Thai patients with DVT.
Keywords: Warfarin, VKORC1, CYP2C9, Deep vein thrombosis (DVT), Single nucleotide polymorphism