The aim of this study is to investigate the in-vivo variability of hemostasis between patients when treated with a direct thrombin inhibitor (Dabigatran) or a direct Factor Xa (FXa) inhibitor (Rivaroxaban) by measuring thrombin generation. In addition, we want to investigate whether there is a correlation between the in-vivo effect and ex- vivo effect in a patient by spiking blood of patients before treatment with rivaroxaban or the active metabolite of dabigatran.