The bleeding in cardiac surgery remains a significant clinical problem. There is no “gold standard” method to quantify blood loss. Traditional measurement of drainage volume often underestimates or overestimates, as it does not consider the type of fluid. We hypothesized that blood loss could be more accurately calculated using the Hb/kg index in terms of haemoglobin (Hb) mass loss per kilogram of the patient’s body mass.

To develop a novel approach for calculating actual blood loss using the Hb/kg index.

This single-center prospective study included 195 patients who underwent cardiac surgery between October 2023 and November 2024. The Hb/kg index was calculated based on intraoperative Hb loss, Hb loss via chest tubes, packed red blood cell transfusions and patient weight. Eighty-six additional clinical predictors were analyzed using conventional statistics and machine learning algorithms. Predictors with statistically significant Spearman correlations were included for further analysis.

Lasso regression achieved the best overall performance in predicting Hb/kg index. It yielded the lowest mean squared error (0.08 ± 0.04), mean absolute percentage error (0.18 ± 0.10), with the highest correlation (0.92 ± 0.06) and R² score (0.82 ± 0.13). BMI showed a significant negative relationship (−0.018, p < 0.001). Postoperative Hb and haematocrit values had negative correlation (−0.69, p < 0.001 and −0.07, p < 0.015), while initial Hb was positively correlated (0.85, p < 0.001).

This method provides a more reliable and clinically relevant tool to calculate actual blood loss and allows for a more precise assessment and treatment.