Which anticoagulant's effect is monitored by the International Normalised Ratio (INR) to guide therapy?

• A. Apixaban
• B. Heparin
• C. Rivaroxaban
• D. Warfarin

Answer: (D)

INR monitoring is used with warfarin, a vitamin K antagonist. Warfarin lowers the production of clotting factors II, VII, IX, and X, and the level of its anticoagulant effect varies from person to person because of diet, drug interactions, liver function, and genetics. Because of this variability, clinicians use the prothrombin time test and convert it to the International Normalised Ratio to standardize results across labs and guide dosing. The target INR range depends on the condition being treated, but a common range is about 2.0 to 3.0, with higher ranges for certain situations like mechanical heart valves. Regular INR checks help keep anticoagulation effective while reducing bleeding risk.

The other options don’t rely on INR for monitoring: direct oral anticoagulants such as apixaban and rivaroxaban are given in fixed doses and don’t require routine INR testing; heparin is typically monitored with the aPTT or anti‑Xa activity, not INR.