Kofi, a 68-year-old man with a mechanical mitral valve, has been on warfarin for two years. He comes to the anticoagulation clinic and mentions he started taking a new herbal supplement for joint pain three weeks ago and has noticed several bruises on his forearms without any known trauma. His INR today returns at 5.8 (therapeutic range 2.5 to 3.5 for his valve). What is the most likely explanation for this finding?
- The bruising and elevated INR indicate his mechanical valve is failing
- Warfarin's effect is wearing off and he needs bridging with heparin
- The herbal supplement likely potentiated warfarin's anticoagulant effect
- His body has developed a tolerance to warfarin requiring a dose increase