Duration of cardiopulmonary bypass is an important confounder when using biomarkers for early diagnosis of acute kidney injury in cardiac surgical patients

Objectives: Acute kidney injury is an important complication in patients undergoing cardiac surgery. Recently, several biomarkers to facilitate early detection of acute kidney injury have been proposed, among them Neutrophil-gelatinase-associated-lipocalin, Kidney-injury-molecule-1, and L-Fatty-acid-binding-protein. The expression of these markers is increased by ischemia-reperfusion injury and may thus be related to the duration of cardiopulmonary bypass (CPB). The present study was designed to determine the effects of CPB-duration on the postoperative course of acute kidney injury biomarkers in comparison with patients developing acute kidney injury according to the classical definition of a decrease in creatinine clearance in a cohort of cardiac surgery patients. Methods: Prospectively sampled data from 136 consecutive patients were analyzed retrospectively. Plasma and urine for determination of biomarkers and creatinine were collected at predefined time points before, immediately after, and up to three days after surgery. The median duration of CPB was 118 min. 29 patients developed acute kidney injury as defined by the acute kidney injury network creatinine criteria. The patients without acute kidney injury were grouped into a "CPB-short" (CPB<118min; n=51) and a "CPB-long" (CPB≥118min; n=56) group. Results: Preoperative plasma Neutrophil-gelatinase-associated-lipocalin and urinary L-Fatty-acid-binding-protein levels were higher in the acute kidney injury than in the CPB-short group. Early postoperative levels of plasmatic and urinary Neutrophil-gelatinase-associated-lipocalin and urinary L-Fatty-acid-binding-protein increased significantly after CPB in all groups and were either higher or not different in comparison between the CPB-long and the acute kidney injury group in the immediate postoperative period. Conclusion: Duration of CPB is a relevant factor for the expression of several biomarkers of renal tubular injury presumed to facilitate early detection of acute kidney injury after cardiac surgery. This questions the usefulness of the markers used in this study for early detection of renal dysfunction and prediction of a further decrease in glomerular filtration rate after cardiac surgery and suggest that CPB-time has to be taken into account when defining cut-off levels for acute kidney injury biomarkers in this setting.