Which parameter is used to monitor warfarin therapy, as illustrated in the patient’s case?

Which parameter is used to monitor warfarin therapy, as illustrated in the patient’s case?

• A. PT
• B. INR
• C. aPTT
• D. Platelet count

Answer: (B)

Monitoring warfarin therapy relies on the International Normalized Ratio (INR). Warfarin disrupts vitamin K–dependent clotting factor production, which extends the prothrombin time (PT). However, PT results vary a lot because different labs use different thromboplastin reagents. The INR standardizes PT results by accounting for the reagent’s sensitivity (the ISI), allowing consistent assessment of anticoagulation across laboratories and over time. Clinically, the goal is to keep the INR within a therapeutic range appropriate for the condition being treated (often around 2.0–3.0, with some situations requiring higher targets). The PT alone isn’t reliable for monitoring because of inter-lab variability, the aPTT is used to monitor heparin therapy (not warfarin), and platelet count doesn’t measure warfarin’s effect.

Monitoring warfarin therapy relies on the International Normalized Ratio (INR). Warfarin disrupts vitamin K–dependent clotting factor production, which extends the prothrombin time (PT). However, PT results vary a lot because different labs use different thromboplastin reagents. The INR standardizes PT results by accounting for the reagent’s sensitivity (the ISI), allowing consistent assessment of anticoagulation across laboratories and over time. Clinically, the goal is to keep the INR within a therapeutic range appropriate for the condition being treated (often around 2.0–3.0, with some situations requiring higher targets).

The PT alone isn’t reliable for monitoring because of inter-lab variability, the aPTT is used to monitor heparin therapy (not warfarin), and platelet count doesn’t measure warfarin’s effect.