Figure 1

Calibration of thromboplastins to determine the Rivaroxaban sensitivity index (here referred as Riva-SI ). Aliquots of a pooled normal plasma were spiked with increasing amounts of rivaroxaban. Plasmas were then tested with five commercial thromboplastins and with a common thromboplastin (referred as common standard). Prothrombin time (PT) results (seconds) were then plotted on a double-log scale (common standard on the vertical axis) and the best-fit line was drawn. The slope of the line, estimated by the orthogonal regression analysis is the Riva-SI and can be taken as a measure of the responsiveness of the rivaroxaban-induced PT prolongation of the five thromboplastins relatively to the common standard. The Riva-SI can be used to convert PT results (seconds) into rivaroxaban standardized PT ratio (here referred as Riva-PT-ratio) according to the following equation: Riva-PT-ratio = [PT _patient /PT _normal ]^Riva-SI. This system of standardization was effective in minimizing the between-thromboplastin variability of the PT for rivaroxaban-spiked plasmas (see text more details). The Riva-SI relatively to the common standard is reported on each graph. By definition the higher the Riva-SI the lower the responsiveness of the thromboplastin to the effect induced by rivaroxaban. The numbers in brackets represent the coefficient of variation of the slope estimation. The Riva-SI for the common standard has been arbitrarily set at 1.00. Results did not change appreciably when Neoplastin Plus was used as the common standard [9].