Prior studies by others have indicated that the thrombin generation(Xi, Beguin et al. 1989; Brummel, Paradis et al. 2001) and antithrombotic influence of factor VII in rabbits (Zivelin, Rao et al. 1993) may be less important than that of prothrombin or factor X. Based on those observations, it is possible that monitoring VKA and their safety could be improved by measuring either coagulation factor II or coagulation factor X. Measuring and monitoring coagulation factor II alone (native prothrombin antigen) has been shown to be very successful and more accurate than PT in clinical studies (Furie, Liebman et al. 1984; Xi, Beguin et al. 1989; Furie, Diuguid et al. 1990; Kornberg, Francis et al. 1993). The native prothrombin antigen assay, however, is not as convenient for laboratories as the PT or P&P. Monitoring coagulation factor X alone has not been tested in patients to our knowledge.