A patient on warfarin therapy presents with bruising and nosebleeds. Which laboratory test result would be most indicative of warfarin overdose?

Choice A reason: An elevated platelet count does not indicate warfarin overdose, which prolongs clotting time by inhibiting vitamin K-dependent factors, not platelet function. Normal or low platelets may cause bleeding, but INR is the key indicator, making this incorrect for assessing overdose. Choice B reason: Elevated aPTT reflects heparin’s effect, not warfarin, which primarily affects prothrombin time and INR. While bleeding may occur, aPTT is not the primary test for warfarin overdose, making this incorrect, as INR directly monitors warfarin’s anticoagulant effect and overdose risk. Choice C reason: An elevated lipid panel is unrelated to warfarin overdose, which causes bleeding by inhibiting clotting factors. Lipid levels affect atherosclerosis, not coagulation, making this incorrect, as INR is the specific test to assess warfarin’s therapeutic and overdose effects in bleeding patients. Choice D reason: An elevated INR level indicates warfarin overdose, as warfarin inhibits vitamin K-dependent clotting factors, prolonging prothrombin time and increasing bleeding risk like bruising and nosebleeds. Monitoring INR is critical, making this the correct test to identify overdose in this patient.