Acute kidney injury (AKI) is a common and serious complication in critically ill patients. The mortality rate in AKI patients remains high despite significant advances in medical care [1-4]. Recently, the Acute Dialysis Quality Initiative (ADQI) work group, comprised of experts in the fields of nephrology and critical care medicine, proposed a multilevel classification system for AKI, the "RIFLE" classification [5], which stands for the increasing severity classes Risk, Injury, and Failure, and the two outcome classes Loss and End-stage kidney disease (Fig. 1). Several studies have shown the RIFLE classification to be a simple, readily available clinical tool to classify acute kidney injury in different populations [6].

In addition to the RIFLE classification, several instruments for scoring illness severity, such as the Sequential Organ Failure Assessment (SOFA), Acute Physiology And Chronic Health Evaluation (APACHE) II and III, and Simplified Acute Physiology Score (SAPS) II, are widely used in critically ill patients; they weigh kidney dysfunction variously [2]. Among these scoring systems, the SOFA score may be used to describe the time course of organ dysfunction in individual patients and is useful to evaluate morbidity [7].

Risk factors associated with hospital mortality in critically ill patients with AKI have been evaluated in several studies. The main risk factors identified were advanced age, use of vasopressors, mechanical ventilation, septic shock, hepatorenal syndrome, and cardiogenic shock [6,8].

The present study was performed to apply the RIFLE classification to patients in intensive care units (ICUs), to identify risk factors related to the development of AKI, and factors associated with hospital mortality in ICU patients.

We conducted a single-center study with 378 ICU patients to characterize AKI, defined by RIFLE classification, and related this classification to mortality in critically ill patients. In the present study, 41.3% of the ICU patients developed AKI, which was associated with increased mortality, compared with those who did not develop AKI. Hoste et al. [2] reported that AKI occurred in 67% of ICU patients, with maximum RIFLE classes of risk, injury, and failure in 12%, 27%, and 28% of cases, respectively. The incidence of almost 70% may appear inconsistent with the existing literature [5,11-13]. The incidence of AKI in the present study was somewhat lower than those in other studies [2,14] using the same RIFLE classification. The most likely explanations are differences in the patient case mix, factors related to a single-center versus multicenter study, and differences in the type of admission (i.e., medical vs. surgical). Hoste et al. [2] suggested that medical admission was less likely to result in AKI, compared with surgical admission [4]. Medical admission accounted for less than 50% of cases in a multicenter cohort study of AKI [4], and the rate was about 78% in a single-center cohort study [14]. In the present study, medical admission accounted for the majority of ICU patients. Hoste et al. [2] reported that increasing age, greater severity of illness (APACHE III, SOFA), preexisting chronic kidney insufficiency, and previous admission to a non-ICU ward were associated with significant mortality rates, ranging from 20% to 80%, and associated with increased risk for the occurrence of AKI.

Importantly, even a mild degree of kidney dysfunction, RIFLE class risk or injury, was associated with elevated mortality rate, compared with patients who maintained normal function. In the present study, the overall hospital mortality rate of ICU patients was 25.7%, which was significantly higher for AKI patients versus non-AKI patients. We also found that ICU mortality ranged from 29% in risk patients, 36% in injury patients, and 51% in failure patients. Factors associated with mortality in our ICU patients included advanced age, development of AKI, baseline severity of illness, and the presence of pulmonary disease and malignancy. These findings were consistent with previous reports [2,3,15]. Hoste et al. [2] demonstrated the association of AKI with increased hospital mortality in a general ICU population and demonstrated that there was increasing mortality risk over RIFLE classes, although these ICU patients had similar comorbidity rates, as reflected by non-renal SOFA scores. They also proposed that the pathophysiological changes resulting from kidney insufficiency and adverse effects of renal replacement treatment may be associated with increased mortality rate [2,16,17]. Uchino et al. [3] reported that there was an almost linear increase in hospital mortality rate with increasing RIFLE class, with patients in the risk group having a mortality rate more than three times higher than that of patients without AKI. In multivariate analysis, the risk group showed an odds ratio of hospital mortality of 2.5, that of the injury group was 5.4, and that of the failure group was 10.1 [3]. They suggested that the development of AKI has greater prognostic implications than the need for ICU or mechanical ventilation, and the development of failure is one of the most powerful identifiable outcome predictors for hospitalized patients [3]. Abosaif et al. [18] also reported that ICU mortality rate increased from 38.3% among patients in the risk group to 50% in the injury group and to 74.5% among patients in the failure group. Our study confirmed that even a mild degree of kidney dysfunction posed a significant risk of hospital mortality.

This study has some limitations. First, it was a retrospective study using existing medical records, which in itself caused some difficulty in the assessment of AKI and the statistical analyses. Second, we classified patients according to the highest RIFLE criteria during the first 7 days in the ICU, similar to the study reported previously by Hoste et al. [2]. More recently, the Acute Kidney Injury Network (AKIN) group proposed refinements to the RIFLE classification [13]. In particular, the AKIN group sought to increase the sensitivity of the RIFLE classification by setting a 48-hour window on the first documentation of criteria and broadening the risk category of RIFLE to include increases in serum creatinine of ≥ 0.3 mg/dL even if this does not reach the 50% cutoff [13].

In conclusion, in these ICU patients, acute kidney risk, injury, and failure, as defined by the recently developed RIFLE classification, were associated with increased hospital mortality rates. The RIFLE classification is a simple and useful clinical tool to detect and stratify the severity of AKI, and possibly to predict outcome.