Activated clotting time (ACT) is commonly used to monitor anticoagulation during cardiac surgeries. Final ACT values may be essential to predict postoperative bleeding and transfusions, although ideal values remain unknown. Our aim was to evaluate the utility of ACT as a predictor of postoperative bleeding and transfusion use.

Retrospective study (722 patients) submitted to surgery between July 2018–October 2021. We compared patients with final ACT < basal ACT and final ACT ≥ basal ACT and final ACT < 140 s with ≥140 s. Continuous variables were analysed with the Wilcoxon rank-sum test; categorical variables using Chi-square or Fisher’s exact test. A linear mixed regression model was used to analyse bleeding in patients with final ACT < 140 and ≥140. Independent variables were analysed with binary logistic regression models to investigate their association with bleeding and transfusion.

Patients with final ACT ≥ 140 s presented higher postoperative bleeding than final ACT < 140 s at 12 h (P = 0.006) and 24 h (**P = 0.004). Cardiopulmonary bypass (CPB) time [odds ratio (OR) 1.009, 1.002–1.015, 95% confidence interval (CI)] and masculine sex (OR 2.842,1.721–4.821, 95% CI) were significant predictors of bleeding. Patients with final ACT ≥ 140 s had higher risk of UT (OR 1.81, 1.13–2.89, 95% CI; P = 0.0104), compared to final ACT < 140 s. CPB time (OR 1.019,1.012–1.026, 95% CI) and final ACT (OR 1.021,1.010–1.032, 95% CI) were significant predictors of transfusion. Female sex was a predictor of use of transfusion, with a probability for use of 27.23% (21.84–33.39%, 95% CI) in elective surgeries, and 60.38% (37.65–79.36%, 95% CI) in urgent surgeries, higher than in males.

Final ACT has a good predictive value for the use of transfusion. Final ACT ≥ 140 s correlates with higher risk of transfusion and increased bleeding. The risk of bleeding and transfusion is higher with longer periods of CPB. Males have a higher risk of bleeding, but females have a higher risk of transfusion.