Which drug's use for anticoagulation is closely associated with INR monitoring?

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

Answer: (A)

INR monitoring is used for anticoagulants whose effect varies greatly between individuals and needs careful dose adjustment. Warfarin blocks vitamin K–dependent production of clotting factors II, VII, IX, and X (and proteins C and S), giving it a narrow therapeutic window. Because responses shift with diet, other drugs, and liver function, clinicians routinely measure the International Normalized Ratio to keep the blood’s clotting tendency within a safe target range. Enoxaparin, clopidogrel, and aspirin do not rely on vitamin K–dependent factors in the same way and are not managed with INR monitoring; enoxaparin is sometimes checked with anti-Xa levels in special cases, while the others are antiplatelet agents. Therefore, the drug associated with INR monitoring is warfarin.