CYP2C9 and atrial fibrillation: Furthermore, analyses of the warfarin arms from the ENGAGE AF-TIMI 48 trial [18] and the Hokusai-venous thromboembolism trial [19] showed that the risk of bleeding was highest in those patients who carried variant alleles for CYP2C9 and/or VKORC1 where standard dosing protocols were used rather than genotype-guided dosing.