During cardiac surgery with cardiopulmonary bypass (CPB), efficient anticoagulation is required in order to avoid microscopic clot formation or, in worst case, massive lift threatening thrombus formation. Heparin is by far the most common drug used to prevent blood from coagulating during CPB. Activated clotting time (ACT) is a point-of-care test of fibrin clotting time that has to be well prolonged to initiate safe CPB.