The adequacy of anticoagulation during CPB can be assessed by the extent to which heparin attenuates the formation and activation of thrombin. However, the very short half-life of thrombin in the presence of heparin and ATIII prohibits the direct measurement of thrombin concentration while on CPB. The peptide prothrombin fragment 1.2 (PT1.2), which is formed as prothrombin is converted to thrombin, is readily measurable and provides a way of evaluating thrombin formation during CPB(5). One of thrombin's major actions, the conversion of fibrinogen to fibrin, releases another measurable peptide, fibrinopeptide A (FPA), thereby providing a means of assessing thrombin activity during CPB(5).