Cardiac surgery–associated acute kidney injury (CSA-AKI) is one of the most frequent complications after cardiopulmonary bypass (CPB), in which hemolysis plays an important pathophysiological role. Carboxyhemoglobin (COHb) plays a key role in metabolism and heme turnover, but its value as a hemolysis-related biomarker of CSA-AKI has not been fully characterized.

We conducted a single-centre, prospective observational study in adults undergoing on-pump cardiac surgery for a 4-month period. The primary objective was to assess the performance value of COHb to predict CSA-AKI and to compare it with other hemolysis-related biomarkers. COHb, methemoglobin, lactate dehydrogenase (LDH), and total bilirubin were evaluated in the immediately postoperatively. CSA-AKI was assessed according to the KDIGO criteria.

Among 262 included patients, 86 (32.8%) developed CSA-AKI. In ROC analysis, COHb showed the best discriminative performance for CSA-AKI (AUC 0.65, 95% CI 0.58–0.72), compared with methemoglobin, total bilirubin, and LDH. An optimal COHb cut-off of 1.25% yielded a sensitivity of 0.58 and a specificity of 0.65. In multivariable logistic regression, immediate postoperative COHb remained independently associated with CSA-AKI (odds ratio 1.89, 95% CI 1.02–3.49), together with age, preoperative atrial fibrillation, and CPB duration. Higher COHb levels (≥1.25%) were associated with increased incidence and severity of AKI and longer mechanical ventilation, but not with vasopressor duration, ICU length of stay, ICU mortality, or postoperative atrial fibrillation. COHb correlated positively with CPB duration.

Immediate postoperative COHb is a potential predictor of CSA-AKI and reflects the intensity of CPB-related hemolysis. COHb may serve as a biomarker to identify patients at higher risk of hemolysis-related CSA-AKI and to guide targeted renal-protective strategies after cardiac surgery.