Several new oral anticoagulants such as rivaroxaban (which targets Factor Xa) and dabigatran etexilate (which targets thrombin) are in advanced stages of clinical development and are already available for clinical use in some countries. Although these agents do not require routine coagulation monitoring, assays to assess the level of anticoagulation may be of assistance in certain circumstances such as in case of overdose, in patients with a hemorrhagic or thromboembolic event during treatment, or to assess compliance. Moreover, the influence of the new oral anticoagulants on routine coagulation tests must be recognized. The prothrombin time is not suitable for rivaroxaban measurement for several reasons, and the routinely used international normalized ratio for monitoring the vitamin K antagonists cannot be applied to rivaroxaban. Development of universal assays is challenging because the new oral anticoagulants have different targets, and even those with the same target have variable effects on routine coagulation assays. Focusing on rivaroxaban, there is emerging evidence that an anti-Factor Xa assay that uses rivaroxaban-containing plasma calibrators may provide the optimal method for determining plasma rivaroxaban concentrations.