TInternational Normalized Ratio (INR) is the accuracy of the Prothrombin Time (PT) is known to be very system-dependent. The World Health Organization has addressed this system variability problem by (1) the establishment of primary and secondary international reference preparations of thromboplastin and (2) the development of a statistical model for the calibration of thromboplastins to derive the International Sensitivity Index (ISI) and the INR. The INR uses the ISI to equate all thromboplastins to the reference thromboplastin through the following equation: