Background The ACEF II score has been proposed as a parsimonious, alternative, operative mortality risk prediction model for cardiac surgery. External validation is warranted to establish its use. Aim The primary goal was to evaluate the ACEF II model performance for cardiac surgery mortality risk stratification. We also tested the discriminatory power to classify patients in need of prolonged postoperative respiratory support and hospitalisation. Methods We evaluated 743 Cardiac Surgery patients – median age 65 (range 20–80 years), 27.4% females - operated between November 2017 and October 2018. Receiver Operating Curves (ROC) were generated based on a dichotomous outcome, “yes/no”, for intrahospital mortality, prolonged mechanical ventilation time (>24 hours), ICU length-of-stay (>48 hours) and postoperative hospitalisation (>7 days). The ACEF II was compared to the ACEF I and the EuroSCORE II (ESII). The DeLong method was used to test the statistical significance of the difference between the areas under different dependent ROC curves. Results The median ACEF II scores for low risk (= ESII <2%), medium risk (= ESII ≥2–≤5%) and high-risk patients (= ESII >5%) were 1.24 (IQR 1.05–1.505), 1.48 (IQR 1.28–1.928) and 2.240 (IQR 1.560–2.933), respectively. The observed mortality for low risk, medium risk and high-risk patients were 1.48% (5/337), 3.26% (9/275) and 19.23% (25/130), respectively. ACEF II outperformed the ACEF I but was similar to the EuroSCORE II in discriminating intrahospital mortality cases and patients in need of prolonged mechanical ventilation (Table 1). All risk models lacked sufficient power to classify patients requiring prolonged ICU-LOS and postoperative hospitalisation time (AUC <0.7). Table 1. Pairwise comparison of ROC Risk Score Model AUC + CI95% – Intrahospital Mortality Area difference when compared to ACEF II AUC + CI95% p-value ACEF II 0.766 [0.733 to 0.796] ACEF I 0.645 [0.609 to 0.679] 0.121 [0.0288 to 0.212] 0.0100 EuroSCORE II 0.809 [0.778 to 0.836] 0.0429 [−0.0431 to 0.129] 0.3284 AUC + CI95% – Prolonged MVT Area difference when compared to ACEF II AUC + CI95% p-value ACEF II 0.721 [0.687 to 0.753] ACEF I 0.632 [0.596 to 0.667] 0.0891 [0.0224 to 0.156] 0.0088 EuroSCORE II 0.721 [0.687 to 0.753] 0.000128 [−0.0732 to 0.0735] 0.9973 AUC = Area Under the Curve, DeLong et al., 1988 – Binomial exact CI95% for the AUC, MVT = Mechanical Ventilation time. Conclusion The ACEF II risk model has a fair discriminative capacity to classify intrahospital mortality cases and patients who will require prolonged mechanical respiratory support following cardiac surgery. Acknowledgement/Funding None