Although the benefit of early RRT initiation on survival outcome was revealed by a recent systemic review and meta-analysis [9], the question of ‘how early is early enough?’ is still unanswered because the early versus late RRT were defined by variable cutoff values of various metabolic parameters such as nitrogenous waste products, sCr, sK+ [34], urine amount, or even clinical judgment alone [9,35]. The present study defines the timing of RRT initiation by using RIFLE classification because this has been extensively validated to standardize the severity of AKI [33].As it is reasonable that the patient survival is artificially extended if it is measured at an earlier time point with better residual renal function and less severity scores, and the so-called survival benefit from early RRT could be accounted for by lead-time bias [34]. However, the period from hospital admission to RRT initiation, as well as the severity scores including APACHE II score and SOFA score and almost all clinical parameters upon RRT initiation, which was taken as a starting point to calculate survival period, were of no statistical differences between ED and LD groups (Table (Table2).2). Therefore, the argument of lead-time bias would be minimized in the current study.Two recent published studies [27,28] have evaluated the association between the timing of RRT initiation by the RIFLE classification and outcome. Neither of them propose clearly defined indications for RRT. Only 33% patients in one study [28] and none in the other [27] were categorized using both GFR and urine output criteria. The retrospective observational study by Li and colleagues [27] enrolled 106 critical AKI patients treated with continuous RRT. It found that the RIFLE classification may be used to predict 90-day survival after RRT initiation, and further analysis revealed that patient in RIFLE-F had a RR of 1.96 (95% CI: 1.06-3.62) comparing with those in RIFLE-R. The predictive effect was also seen in our work in which the RR of sRIFLE-F to sRIFLE-R was 3.194 (P = 0.014). As to the study of Maccariello and colleagues [28], a prospective cohort study including 214 AKI patients who underwent RRT, the RIFLE classification didn’t show discrimination of prognosis in all patient populations. However, the association between RIFLE-F and increased in-hospital mortality was found while conducting a separate analysis study using only patients who underwent ventilation and vasopressors.