Why is INR used only in patients taking warfarin and not other anticoagulants?

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  • 2 months ago
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    "Warfarin decreases blood clotting by blocking an enzyme called vitamin K epoxide reductase that reactivates vitamin K1. Without sufficient active vitamin K1, clotting factors II, VII, IX, and X have decreased clotting ability. The anticlotting protein C and protein S are also inhibited, but to a lesser degree."

    "The prothrombin time is a measure of the integrity of the extrinsic and final common pathways of the coagulation cascade. This consists of tissue factor and factors VII, II (prothrombin), V, X, and fibrinogen"

    The INR or PT is used to measure the effects of Warfarin vitamin K dependent coagulation factors and activity. The PT test system is optimized to measure activity along a linear curve and that is important when one wants to see gradual changes rather than non-linear abrupt changes as the Warfarin concentration changes. You want to see a gradual increase in PT with a similar gradual increase in Warfarin like if you double the concentration then you want to see a doubling in the PT time. It becomes difficult when it comes to dosing if there isn't a linear relationship that one can count on. 

    The PTT is optimized for heparin monitoring, more specifically the activated factor Xa concentration.

    The PT INR is insensitive for monitoring heparin concentration. It is not intended for that. 

    There's newer anticoagulants where the PT and PTT are insufficient and insensitive for use in monitoring such anticoagulants. 

    There's acquired medical conditions where the PT and or PTT are inadequate in monitoring Warfarin and or heparin. In such cases other tests must be used. 

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