A young male developed spontaneous DVT of right lower limb and he is having a sister who had DVT 5 years back and having protein C deficiency. He was started on warfarin and the INR came to 2-3 after which he had started following at local hospital where his warfarin dose was steadily increased as his INR never went higher than 1.5 and is currently on warfarin dose of 20 mg/day. He was referred back to hospital and was evaluated and found to be having warfarin level of 2.385 mg/l (therapeutic range 0.7 – 2.3 mg/l), PIVKA is > 10 (Ref. range < 0.2). What is the most likely explanation for the subtherapeutic INR?
a. Cytochrome p450 mutation
b. VKORC 1 mutation
c. Not taking warfarin
d. Local hospital INR testing quality control is not good