Apixaban Versus Rivaroxaban in Non Valvular Atrial Fibrillation

The investigators will first measure the maximum concentration (after 2 hours of intake) and the residual concentration (just before the next intake) after at least 15 consecutive days of treatment. In order to be able to study the stability of the anti-Xa activity of Apixaban vs Rivaroxaban, as well as their impact on the risk of thromboembolic events or hemorrhagic events, clinical follow-up and a determination of maximum and residual activity are necessary, ideally at 3 to 6 months (compared to studies carried out in the literature). This evaluation would be made according to a multivariate analysis taking into consideration the other clinical-biological data relating to the patient, namely renal function, liver function, CHA2DS2-VASc score, HAS-BLEED score, treatment compliance, etc.