Excessive bleeding following cardiac surgery involving cardiopulmonary bypass (CPB) is common, and it may be difficult to determine whether the bleeding is due to surgery, to medications for prevention of clotting, or to primary blood clotting disorders. Platelets are an essential component of blood clotting and they can be damaged during passage through the CPB machinery. Assessment of platelet function may help clinicians determine the cause of bleeding, which could reduce unnecessary transfusions. Although there are a variety of platelet function and general blood clotting tests (standard laboratory tests or SLTs), the turnaround time for many of these tests may be too long for the results to be clinically useful during or right after surgery.