To determine whether low pre- or post-bypass antithrombin (AT) activity identifies adult cardiac surgery patients at increased risk of adverse outcomes in settings where plasma transfusion is rare.

Prospective exploratory observational cohort.

Two academic cardiac surgery centers using factor concentrate-based hemostatic algorithms without routine AT supplementation.

One hundred three adults (≥18 years) undergoing cardiac surgery with cardiopulmonary bypass.

Fibrinogen concentrate and 4-factor prothrombin complex concentrate (4F-PCC) were first-line hemostatic agents. AT activity was measured immediately before and after cardiopulmonary bypass.

Two patients (2%) received plasma, 23% received 4F-PCC, and 26% received fibrinogen concentrate. Prebypass AT activity was categorized, comparing the lowest quintile (Q1) with higher quintiles (Q2-Q5). Co-primary outcomes were hospital and intensive care unit length of stay to postoperative day 28. Secondary outcomes included acute kidney injury, composite major morbidity, and vasopressor or inotrope use beyond postoperative day 1. Q1 patients had lower postbypass AT levels and higher rates of acute kidney injury, vasopressor/inotrope use, and longer hospital stay, but prebypass AT status was not an independent predictor in multivariable models. A sensitivity analysis using a postbypass AT threshold level of 63.7% showed similar results.

In cardiac surgery patients where plasma was avoided and no AT supplementation was used, low perioperative AT activity was associated with higher unadjusted complication rates but did not independently predict outcomes after accounting for patient and procedural factors. The routine use of factor concentrates over plasma as first-line hemostatic therapy, without additional routine AT supplementation, does not appear to increase risk.