Early versus late renal replacement therapy in acute kidney injury: the search for a definition of timing continues

None of the studies reviewed used the Acute Kidney Injury Network (AKIN) criteria [3]. Having applied the AKIN criteria to data from our 20-bed critical care unit for all patients receiving RRT in 2009 (n = 158), we also failed to show RRT initiated at an early stage of AKI to be of benefit in terms of critical care or hospital survival. Figure 1 shows a higher rate of hospital mortality in patients with AKIN stage 1 AKI (P = 0.01). Patient characteristics including age, sex and Acute Physiology and Chronic Health Evaluation scores were similar across all groups. In view of our experience and Karvellas and colleagues' conclusions, surely the matter of greatest urgency prior to any multicentre trial is establishing a satisfactory definition for early and late initiation of RRT.Constantine J Karvellas and Sean M BagshawWe appreciate the comments by Bannard-Smith and Mousdale pertaining to our systematic review of early versus late initiation of RRT in critically ill patients with AKI [1]. Indeed, we found that the available literature to date is prohibitively heterogeneous, in particular with respect to study methodology, study quality and operational definitions of timing of RRT.As mentioned, few studies included in our review utilised the RIFLE criteria to define timing. One study compared timing by fulfilling RISK versus INJURY or FAILURE categories in medical ICU patients [4], while the other compared RISK and INJURY versus FAILURE in surgical/trauma patients [5]. While no study specifically utilised the AKIN modifications of RIFLE to define timing, it is unlikely there are substantial differences between these classifications for the discrimination of timing or associated outcomes [6].In part, this is attributed to the similarity of these classifications, both of which rely exclusively on changes in serum creatinine and urine output as defining features. This is clearly suboptimal when considering the complexity of the decision to initiate RRT in critically ill p