Hemolysis Index, Carboxyhemoglobin, and Methemoglobin for the Early Identification of Patients at Risk for Cardiac Surgery-Associated Acute Kidney Injury.

Abstract

Objectives: Hemolysis is a contributor to CS-AKI. Biochemistry analyzers provide a hemolysis index to quantify in vitro hemolysis, a condition that can, for example, affect the accuracy of potassium concentration measurements. We aimed to assess whether the postoperative plasma level of the hemolysis index (HI_{postoperative}) could aid the early recognition of patients at risk for cardiac surgery-associated acute kidney injury (CS-AKI) and also to evaluate other hemolysis indicators: plasma carboxyhemoglobin (COHb_{postoperative}) and methemoglobin (MetHb_{postoperative}).

Design: One-year retrospective study.

Setting: University hospital.

Participants: Patients undergoing elective cardiac surgery.

Interventions: None.

Measurements and main results: In 1090 patients, the median HI_{postoperative} was higher in patients who developed CS-AKI compared to patients who did not (11 mg/dL [interquartile range (IQR), 5-38 mg/dL] v 7 mg/dL [IQR, 3-16 mg/dL]; p < 0.001). HI_{postoperative} refined the early recognition of CS-AKI: the area under the precision-recall curve (AUPRC) for HI_{postoperative} was 37% (95% confidence interval [CI], 31%-42%), whereas the AUPRC associated with no discriminative power, equal to the prevalence of CS-AKI in the whole population, was 21%. Among the 611 patients with measurements for all 3 biomarkers, the AUPRC of HI_{postoperative} was higher than that of COHb_{postoperative} or MetHb_{postoperative} (+6.6% and +7.4% respectively; p < 0.0001 for both). Unlike COHb_{postoperative} or MetHb_{postoperative}, the incorporation of HI_{postoperative} into a model (trained on a sample then validated in another sample) of CS-AKI early recognition significantly enhanced its performance, with a +1.9% (95% CI, 1.6%-2.1%) increase in AUPRC (p < 0.0001).

Conclusions: Elevated HI_{postoperative} represents an early alert signal for the development of CS-AKI. Our findings support the incorporation of HI_{postoperative}, a readily available biomarker, into predictive scores of CS-AKI.