Non-vitamin K antagonist oral anticoagulants (NOACs) are recommended over warfarin in preventing stroke and thromboembolism among patients with atrial fibrillation (AF) in several guidelines. To evaluate the pharmacological effects of NOACs, directly measuring the concentration is the most arbitrary way since the correlation between concentration and common coagulation tests are not reliable. Our previous investigation reported under the fixed dose regimen, dabigatran exposure increased in elderly, renal impairments and patients with multiple co-morbid conditions. Our data also showed difference in NOACs exposure in Asians. For example, patients under rivaroxaban, in comparison to apxiaban, were more likely to have lower than expected range drug level. Furthermore, the NOACs concentration also affected by the prescription pattern of physicians (non-compliant to labeled dose) and patients' behavior (poor medication adherence). The relationship between NOACs exposure and safety has been elucidated in large-scale clinical trials. As the NOACs level increased, the risk for bleeding increased, too. Nevertheless, no additional protection was noted with increased NOACs levels. In post marketing surveillance, bleeding and thrombotic events have been reported. Investigating the NOACs level among these patients helps evaluating the residual drug in the body, which could be a reference for clinical decision in emergent situation. Specific purpose: Investigate the correlation between NOACs concentration upon the arrival of emergency department (ED) and important clinical outcomes including systemic thromboembolism, and major bleeding. Direction for investigation: 1. Prospectively record the NOACs concentration among AF patients under NOACs therapy and suffered from ischemic stroke (IS), transient ischemic attack (TIA), intracerebral hemorrhage (ICH) and other major bleeding. 2. Investigate the correlation between NOACs concentration upon ED arrival and thromboembolic or bleeding events. 3. Propose a therapeutic range for NOACs, in order to provide a guide for important decision in acute setting.