Introduction. The RIFLE classification defines three severity criteria for acute kidney injury (AKI): risk, injury, and failure. It was associated with mortality according to the gradation of AKI severity. However, it is not known if the APACHE II score, associated with the RIFLE classification, results in greater discriminatory power in relation to mortality in critical patients. Objective. To analyze whether the RIFLE classification adds value to the performance of APACHE II in predicting mortality in critically ill patients. Methods. An observational prospective cohort of 200 patients admitted to the ICU from July 2010 to July 2011. Results. The age of the sample was 66 (±16.7) years, 53.3% female. ICU mortality was 23.5%. The severity of AKI presented higher risk of death: class risk (RR = 1.89 CI:0.97–3.38, ), grade injury (RR = 3.7 CI:1.71–8.08, ), and class failure (RR = 4.79 CI:2.10–10.6, ). The APACHE II had C-statistics of 0.75, 95% (CI:0.68–0.80, ) and 0.80 (95% CI:0.74 to 0.86, ) after being incorporated into the RIFLE classification in relation to prediction of death. In the comparison between AUROCs, . Conclusion. The severity of AKI, defined by the RIFLE classification, was a risk marker for mortality in critically ill patients, and improved the performance of APACHE II in predicting the mortality in this population. 1. Introduction The epidemiology, evolution, and treatment of acute kidney injury (AKI) in critically ill patients were better evaluated only after the introduction of Intensive Care Units (ICU) and the introduction of dedicated critical care medicine journals in the 1970s . However, only since the 1980s, scores of disease severity were developed. Those scores were perfected in the 1990s, highlighting the Acute Physiology and Chronic Health Evaluation (APACHE) , the Therapeutic Interventions Scoring System (TISS) , and the Sequential Organ Failure Assessment (SOFA) . The APACHE II  is the most often cited model in medical literature and the most used nowadays, being recommended by a ministerial order in Brazil since 1998 . These prognostic models are used in the ICU to predict the outcome of patients with certain severe diseases, including acute kidney injury, and the APACHE II score has been the most commonly used predicting instrument in this population . The work done that evaluated the power of APACHE II in predicting the mortality found values ranging from 0.75 to 0.90 [6, 8, 9] which were considered excellent. However, the results of the analyses of their performance in subgroups are controversial
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