The rise and fall of the K(t)/V concept in CAPD.

A simple figure to quantify the dialysis dose is attractive by all means. It facilitates the dialysis prescription and allows to study effects of the dialysis dose on outcome. KtuVurea as developed by Gotch et al. for haemodialysis, meets this requirement because essentially it means that large patients need more dialysis, either by a higher urea clearance or by a longer treatment time, than small patients do w1x. The disadvantage of KtuVurea is that only the clearance of urea is taken into account, one of the smallest uraemic toxins, while no attention is paid to the removal of the so-called ‘middle-molecules’ and of excess of body fluid. Peritoneal dialysis is different from haemodialysis, because the peritoneal clearance of low molecular weight solutes is much lower than in haemodialysis while that of middle-molecules is higher. For instance, the average peritoneal urea clearance in CAPD is 7 mlumin, which is about half of that delivered on a weekly basis by haemodialysis. However, the peritoneal clearance of b2-microglobulin ranges between 0.4 and 1.3 mlumin w2x. These values are much higher than those achieved by conventional haemodialysis w3x. Nevertheless urea kinetic modelling has also been advocated for the assessment of adequacy in CAPD w4x. In this comment the first retrospective studies on KtuVurea in peritoneal dialysis will be discussed, followed by prospective cohort studies and studies in anuric peritoneal dialysis patients. Finally these studies will be compared to those obtained in a randomized controlled trial.