Which drug is primarily monitored with INR to ensure therapeutic anticoagulation in patients?

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

Answer: (B)

INR monitoring is used for drugs that require regular dose adjustments because their effect on clotting can vary a lot between people. Warfarin is a vitamin K antagonist that reduces the synthesis of clotting factors II, VII, IX, and X, which makes its anticoagulant effect highly variable due to diet, other medications, and genetics. The PT is sensitive to these changes, and the INR standardizes this measurement across labs, so clinicians can reliably adjust the Warfarin dose to reach a therapeutic range tailored to the patient's indication (commonly around 2.0–3.0, sometimes higher for certain valve conditions).

Other options do not rely on INR for routine monitoring. Heparin is followed with aPTT or anti-Xa levels, not INR. Direct oral anticoagulants like dabigatran and rivaroxaban have predictable effects and are given in fixed doses without routine laboratory INR checks, except in special situations.