Kt/V urea does not tell it all.

In this issue of Nephrology Dialysis and Transplantation, Sirich et al. provide evidence that 8-h treatments with large dialysers and higher dialysate flows greatly increase the clearances of protein-bound compounds p-cresol sulphate and indoxyl sulphate without altering Kt/V urea [1]. Thus, this paper is a further striking example of the limitations of urea-based dialysis ‘dosing’ and allows us to confirm that Kt/V urea does not tell it all. In order to quantify the removal of uraemic toxins and the treatment time, some decades ago, Sargent and Gotch proposed a mathematical model based on the dialytic removal of urea, introducing the concept of Kt/V urea, i.e. a clearance that is expressed as the product of the dialyser clearance (K) multiplied by the duration of treatment (t) and divided by the estimated volume of distribution (V) of urea [2]. Urea seemed ideal as a marker because it is the end product of protein catabolism, is a small water-soluble molecule (60 Da), easily measured in blood and dialysate and with V equal to the water body volume.