Apixaban peak blood level is about 4 hours. A clot takes a few hours to 48 hours to form in the left atrium. Apixaban taken every 12 hours will prevent more strokes, but it will cause more bleeds. No one has done the study of taking it within 12 hours of afib onset, but I expect with all the monitoring people are doing, it would be a useful study. You are right that thrombolytics are often used in clots that have already formed, but it has to be given pretty quickly to be effective.