Optimizing the verification of mean normal prothrombin time (MNPT) and international sensitivity index (ISI) for accurate conversion of prothrombin time (PT) to international normalized ratio (INR)

Abstract

The Prothrombin Time (PT) assay is the most common test performed in hemostasis laboratories, most commonly as converted to an international normalized ratio (INR) to monitor anticoagulant therapy using vitamin K antagonists (VKAs) such as warfarin. Although the INR is meant to standardize PT values by taking into consideration reagent and instrument variability, substantial inter-laboratory variation in INRs still exists and suggests that this can be further improved. This paper describes the PT test, its conversion to an INR value, and methods to improve the accuracy of INRs by improving the determination of critical components to the INR, namely the mean normal prothrombin time (MNPT) and the international sensitive index (ISI).