Warfarin and the international normalized ratio: reducing interlaboratory effects

In North America, the dose of warfarin has been unintentionally increased during the past two decades because of failure to recognize the effect on the anticoagulation test of less sensitive tissue thromboplastins. Although there still is some controversy, the suggested dose of warfarin has now been adjusted downward to reduce the risk of bleeding. These revised dosing recommendations incorporate the international normalized ratio (INR), which takes into account the source of thromboplastin. However, there is considerable variability in the sensitivities of thromboplastin from manufacturer to manufacturer and lot to lot. Therefore, prothrombin times (PTs) are not comparable from laboratory to laboratory without knowing the sensitivity of the thromboplastin. Unless laboratories adopt a standardized tissue thromboplastin, the PT should be reported as an INR.