how to calculate inr
INR = [PTpatient Ã· MNPT]ISI. The INR has no units (it is a ratio) and is determined to one decimal place. The first step of the INR calculation is to ânormalizeâ the PT by comparing it to the mean normal prothrombin time (MNPT), the geometric mean of the prothrombin times of the healthy adult population. In the second step, this ratio is raised to a power designated the ISI, or international sensitivity index.