[Changes in PT and APTT When Administrating Rivaroxaban, a Direct Inhibitor of Activated Factor X]

The effects of oral rivaroxaban (RX), a direct inhibitor of activated factor X (Xa), on prothrombin time (PT) and activated partial thromboplastin time (APTT) were examined. PT and APTT before and after administration of 15 mg RX in 10 healthy subjects were measured by using various reagents. In addition, the blood Xa inhibitor concentration was measured and its correlation with PT and APTT, as measured by each reagent, was examined. Furthermore, the relationship (sensitivity) between the prolongation ratio and FX activity was evaluated. Prolongation of both PT and APTT was observed after RX administration, and maximal prolongation was observed four hours after administration for each reagent. The prolongation ratio was different among the reagents used for examination, and the reagent with the highest sensitivity to FX and factor VII showed the largest prolongation ratio. PT and APTT were positively correlated with RX concentration when measured by any reagent. In this study, neither PT, APTT, nor RX concentration returned to the values measured prior to dosing even at 24 hours after administration of RX. Our results suggest that approximate concentration of remaining RX may be estimated by PT (second) when using a reagent showing a larger prolongation ratio.