Serum Creatinine Measurement Immediately After Cardiac Surgery and Prediction of Acute Kidney Injury

Background: After heart surgery, acute kidney injury (AKI) confers substantial long-term risk of death and chronic kidney disease. We hypothesized that small changes in serum creatinine (SCr) levels measured within a few hours of exit from the operating room could help discriminate those at low versus high risk of AKI. Study Design: Prospective cohort of 350 elective cardiac surgery patients (valve or coronary artery bypass grafting) recruited in Winnipeg, Canada. Baseline SCr level was obtained at the preoperative visit 2 weeks before surgery. The postoperative SCr level was drawn within 6 hours of completion of surgery and then daily while the patient was in the hospital. Predictor: Immediate (ie, <6 hours) postoperative SCr level change (Delta SCr), categorized as within 10% (reference), decrease >10%, or increase >10% relative to baseline. Outcome: AKI, defined according to the new KDIGO (Kidney Disease: Improving Global Outcomes) consensus definition as an increase in SCr level >0.3 mg/dL within 48 hours or >1.5 times baseline within 1 week. Measurements: We compared the C statistic of logistic models with and without inclusion of immediate postoperative Delta SCr. Results: After surgery, 176 patients (52%) experienced a decrease >10% in SCr level, 26 (7.4%) experienced an increase >10%, and 143 had Delta SCr within +/- 10% of baseline. During hospitalization, 53 (14%) developed AKI. Bypass pump time, baseline estimated glomerular filtration rate, and European System for Cardiac Operative Risk Evaluation (euroSCORE) were associated with AKI in a parsimonious base logistic model. Added to the base model, immediate postoperative Delta SCr was associated strongly with subsequent AKI and significantly improved model discrimination over the base model (C statistic, 0.78 [95% CI, 0.71-0.85] vs 0.69 [95% CI, 0.62-0.77]; P < 0.001). A >= 10% SCr level decrease predicted significantly lower AKI risk (OR, 0.37; 95% CI, 0.18-0.76), whereas a >= 10% SCr level increase predicted significantly higher (OR, 6.38; 95% CI, 2.37-17.2) AKI risk compared with the reference category. Limitations: We used a surrogate marker of AKI. External validation of our results is warranted. Conclusion: In elective cardiac surgery patients, measurement of immediate postoperative Delta SCr improves prediction of AKI. Am J Kidney Dis. 59(2):196-201. (C) 2012 by the National Kidney Foundation, Inc.