**For calculating the international normalized ratio, a patient's prothrombin time is divided by the mean normal prothrombin time.** This ratio is raised to a power called the international sensitivity index.

In this equation, a patient's prothrombin time is determined through a blood test. The mean normal prothrombin time is the geometric mean of the prothrombin times of the healthy adult population. The ISI depends on the thromboplastic reagent and is derived from data from normal, healthy individuals and individuals stabilized on warfarin. The INR was introduced to standardize prothrombin time results that vary by laboratory. INR results are most commonly used to guide warfarin dosing.