Rivaroxaban is the first oral anticoagulant with a direct anti-Xa activity to be registered (approval). As for all first comers in a class, it should be assessed both for itself and for the class. The targeting of factor-Xa factor, key component in the coagulation cascade, has the theoretical benefit of being an effective antithrombotic and a potential risk for hemorrhage, both highly dose-dependent. Experience has shown us that the representativeness and predictiveness of in vitro tests and preclinical models are only partial and sometimes even misleading. This is why the responses can only come from clinical trials and rigorous research testing doses, which should be conducted specifically in all the indications foreseen, with no extrapolations. The oral anticoagulant drugs are developed in the prevention of arterial thromboembolic events caused by atrial fibrillation too, where the vitamin K antagonists (VKAs) are the current standard of care. The well-known problems of monitoring and adaptation doses with VKAs have led to developing new replacement classes without the need for control or biological adaptation. However, in certain conditions there is a need to monitor the patient. The advantage for the direct anti-Xa inhibitors such as rivaroxaban is that the prothrombin time, a routine test is sensitive and provides a prolonged response that is proportional to the plasma concentration within a wide range of concentrations. This test is potentially usable provided that the indispensable standardization is forthcoming.