A retrospective analysis involving 686 patients favored ACT-based monitoring compared with heparin concentration monitoring because of less post- operative bleeding and transfusion requirements associ- ated with ACT-based monitoring [17]. Ann Thorac Surg 2018;105:650 62 ANTICOAGULATION DURING CARDIOPULMONARY BYPASS REPORT Traditionally, the gold standard for measuring the anticoagulant effects of heparin is inhibition of factor Xa (anti-Xa) activity. Factor Xa is a major target for unfrac- tionated heparin and can be readily measured in plasma using laboratory assays. The various studies that seek to validate a new measure of heparin activity, or a clotting time assay, use anti-Xa activity as the gold standard comparison. However, plasma assays for anti-Xa activity are not ideally suited for point-of-care testing. Anti-Xa measurement serves as a validating test for novel point- of-care assays that re ect anti-Xa activity. Hansen and associates [18] studied a whole blood modi ed ACT test and found it to be highly correlated to laboratory anti-Xa measurement. Helstern and associates [19] reported another one-step clotting assay that correlates well with anti-Xa tests and is not in uenced by hemodilution, but clinical studies are lacking. Routine redosing of unfractionated heparin at xed intervals during CPB, despite a therapeutic ACT, is commonly used when heparin concentration assays are not available to simulate the practice of higher heparin dosing. This practice prescribes additional xed doses of unfractionated heparin at speci c timepoints, even though ACT may be above target. In a prospective trial of 100 patients presenting for cardiac surgery, one third of the initial heparin bolus was administered at the 90- minute point of CPB, with repeat doses every 60 mi- nutes thereafter [8]. This strategy maintained adequate anticoagulation during the entire period of hypothermic CPB; bleeding variables were not reported. 