An elderly patient attends surgery with a grossly carious upper molar which has an abscess. Although in pain the tooth cannot be extracted at present as the patient has an INR of 5.5, which medication is most likely causing this?

• A. Aspirin
• B. Warfarin
• C. Clopidogrel
• D. Heparin

Answer: (B)

Warfarin lowers the blood’s ability to clot by inhibiting vitamin K–dependent clotting factors, which lengthens the prothrombin time and is reflected as the INR. An INR of 5.5 shows significant anticoagulation and a high bleeding risk if an extraction is done. Of the options, only Warfarin directly raises the INR. Aspirin and clopidogrel affect platelets but do not change INR, and heparin changes the aPTT rather than the INR. So the medication most likely causing this elevated INR is Warfarin. In practice, a high INR like this would prompt medical review before proceeding with extraction, with possible adjustment or reversal of anticoagulation.