in the previous issue of critical care , an article by vesconi and colleagues   describes the impact of the dose of renal replacement therapy ( rrt ) on clinical outcomes in critically ill patients with acute kidney injury . this study was performed in a number of european intensive care units ( icus ) . dialysis dose was defined as more intensive ( continuous rrt ( crrt ) , > 35 ml / kg / h ; intermittent rrt ( irrt ) , > 6 sessions / week ) or less intensive ( crrt , < 35 ml / kg / h ; irrt , < 6 sessions / week ) . the study did not provide evidence for a survival benefit of a higher dose of rrt . however , more intensive rrt was associated with a favourable effect on icu stay and duration of mechanical ventilation among the surviving patients . studies of this nature are not easy to perform and the authors are to be commended for their initiative . this paper highlights many of the controversial aspects of rrt management in these patients , including when to start , which modality is preferable , and what dose to apply . these controversies are partly fuelled by a lack of consistent data , which are difficult to obtain in this heterogeneous patient group , with wide ranges of applied practices and underlying diseases and the absence of universally accepted definitions . for example , analysis of table 1 in the paper by vesconi and colleagues   reveals the wide diversity of the patient study population involved ; the distribution of the rifle classification at the start of rrt is intriguing as a substantial portion of the patients was started on dialysis without even having class ' i ' of rifle . besides the fact that the rifle criteria are not intended to be a guide as to when to start rrt , this means that some of these patients had near ' normal ' serum creatinine levels at the beginning of rrt . the question can be raised as to whether the inclusion of such patients influenced the outcome , although it is unclear how and to what extent this might be the case . another question to be considered is how the results of this study should be interpreted in the context of other dose of dialysis / outcome randomized trials using crrt ( continuous haemofiltration or haemodiafiltration ) , irrt  including the conventional 3 to 4 hour sessions or the prolonged slow extended dialysis ( sled ) method  or both . some showed a benefit of the ' higher than usual doses ' [ 2 - 4 ] , but others did not [ 5 - 8 ] . all these studies were accompanied by a number of critical reviews and editorial comments [ 9 - 12 ] . although none of the studies showing no benefit of higher dose of rrt were perfect , all have one advantage : they reflect daily practice . the problem is that we have no correct understanding of what ' dose ' means and higher doses implicitly mean more complications and/or comorbidities . in the study by vesconi and colleagues , differences in dose for crrt were not pursued intentionally but seem to have been the consequence of external conditions , of which body weight of the patients was the main one . the patients with the lowest dose of crrt were approximately 33% heavier than those with the highest dose . this is , of course , an important and remarkable confounder . in interventional studies where dialysis dose is normalized to body weight what should be considered as ' body weight ' : dry weight , weight before admission , real ad hoc weight , distribution volume of urea , or something else ? and how do we estimate this sometimes quite fluctuating ' body weight ' in a critically ill patient ? in the irrt group , the dose was expressed as the number of dialysis days per week , targeting a kt / v urea threshold . however , as long as we know neither the nature nor the kinetics of many of the ' toxins ' involved in the acute ' uraemic syndrome ' we will always have to rely on surrogate markers , of which the volume status of the patient  which should be frequently clinically evaluated  and electrolyte and acid - base disturbances  which should be managed are probably the most important . as such , the fact that absolute ultrafiltered volumes were comparable in both the less and more intensive patient groups makes it questionable whether there really was a difference of dose in the study by vesconi and colleagues . interestingly , there was also no difference in obtained net ultrafiltration between low and intensive dose groups in the va / nih trial   and a study by tolwani and colleagues . so , maybe our ' concept ' of what dose actually means in icu patients needs to be redefined . the high dose group in the va / nih study   suffered a substantially higher number of complications , such as haemodynamic instability and electrolyte disturbances . the potential benefit of achieving more clearance can thus be eliminated by a higher complication rate . complication rates might , in turn , be corrected by centre experience ; this could explain why the first , but single centre , study of ronco and colleagues   found that dialysis dose did have an impact whereas the multicentre va / nih study   did not . as the latter study enrolled approximately 500 patients in 30 centres , it can be presumed that a substantial number of the study centres recruited less than 10 patients per year . the ' dose of experience ' with rrt in the icu might be a greater determining factor of outcome in critically ill acute kidney injury patients than the dose of dialysis . finally , despite the previous seminal study   by the senior author of the paper by vesconi and colleagues demonstrating that a ' high ' dose of 35 ml / kg / h crrt is advantageous , the latter study shows that , in daily clinical practice , this dose is quite difficult to achieve . it should be appreciated by readers that the same advocates of a high dose of rrt are now amending their previous recommendations . crrt : continuous rrt ; icu : intensive care unit ; irrt : intermittent rrt ; rrt : renal replacement therapy .
many studies on the most ' adequate ' dose of renal replacement therapy ( rrt ) in critically ill patients with acute kidney injury have obtained contradictory results . 
 the previous issue of critical care reports a multi - centre study showing that a higher than conventional dose of rrt , whether continuous rrt or intermittent rrt , is not associated with better survival of these patients . 
 this commentary highlights some of the problems associated with the interpretation of this and previously published studies . 
 these problems include the use of targets of kt / v urea or the ultrafiltration rate in millilitres per kilogram body weight , the latter quite difficult to estimate in these patients , the divergent co - morbidities of the patients , and the local experience of intensivists or nephrologists with either continuous rrt or intermittent rrt . 
 the last factor could explain why some single centre studies did find an impact of dialysis dose on the survival of these patients whereas multi - centre studies did not .