One of the primary risks associated with LVADs are those related to blood clotting. Maintenance of the LVAD depends on preventing blood clots especially right after the surgery that puts them in place. This requires putting patients with LVADs on blood thinners and then monitoring the blood thinner levels carefully so that they don't have problems with bleeding too easily or, on the other hand, forming blood clots if they aren't on the right dose of blood thinners. Excessive bleeding - called hemorrhaging - is the more common problem after surgery and half of all patients that receive an LVAD will require a blood transfusion within the first 30 days after they receive the device. A laboratory test that measures how quickly blood clots is used to determine if the patient has the right amount of blood thinners. The current standard test is called "activated partial thromboplastin time [aPTT]." There are problems with the reliability of this test because it reacts with other elements in the blood. Alternately, there is a test that more directly measures the amount of heparin blood thinner rather than coagulation time and may be more reliable than aPTT called anti-factor Xa [anti-Xa].