Warfarin is Monitored by INR
The correct answer is C. Warfarin. Warfarin is the anticoagulant monitored by International Normalized Ratio (INR), which was specifically developed and validated to standardize prothrombin time (PT) measurements for patients receiving vitamin K antagonist (VKA) therapy 1.
Why Warfarin Requires INR Monitoring
Warfarin inhibits vitamin K-dependent clotting factors (II, VII, IX, and X), causing prolongation of the PT that must be carefully monitored to balance thrombosis prevention against bleeding risk 1.
The INR standardizes PT measurements across different thromboplastin reagents and laboratories, calculated as INR = (patient PT/mean normal PT)^ISI, where ISI is the International Sensitivity Index of the thromboplastin used 1.
INR was specifically designed and validated using plasma samples from patients on stable VKA therapy, making it the gold standard for warfarin monitoring 1.
Target INR Ranges for Warfarin
Most indications require INR 2.0-3.0, including atrial fibrillation, deep vein thrombosis, and pulmonary embolism 2, 3.
Higher intensity INR 2.5-3.5 is recommended for mechanical heart valves (particularly tilting disk and bileaflet valves in mitral position) and certain high-risk post-MI patients 2, 3.
INR values below 2.0 increase thromboembolism risk, while values above the therapeutic range sharply increase bleeding risk 2.
Why the Other Options Are Incorrect
Unfractionated heparin (UFH) is monitored by activated partial thromboplastin time (aPTT), not INR 1.
Enoxaparin (low molecular weight heparin) typically requires no routine monitoring, though anti-Xa levels can be measured in specific situations like renal insufficiency or obesity—not INR.
Rivaroxaban is a direct factor Xa inhibitor that does not require routine coagulation monitoring and does not affect INR in a predictable, dose-dependent manner suitable for therapeutic monitoring 1.
Critical Monitoring Pitfall
The INR is only valid for warfarin monitoring—it should not be used to assess bleeding risk or guide plasma transfusion in patients not taking VKAs, as it was never designed or validated for these purposes 1. Using INR outside the context of VKA therapy lacks both biological validity and clinical evidence 1.