Mathematical modelling of haemodialysis in children.

The single-pool urea kinetic model (UKM), utilising "Kt/V" (the normalised whole body urea clearance), is widely used to help assess the adequacy of haemodialysis in adults. In the presence of an adequate dietary protein intake, a value of unity is acceptable for thrice weekly dialysis. Children could benefit from this approach but, with their relatively higher protein intakes and dialysis needs, this model may not be applicable. Urea kinetics, studies in six children with chronic renal failure by serial timed blood urea measurements during and after haemodialysis, were compared with the kinetics of a one-pool and a two-pool UKM. The two-pool UKM with intra- and extracellular pools best fitted the observed data, re-equilibration between pools accounting for the marked rebound increase in blood urea seen in the 1st h after dialysis (mu 17%, SD 5). Kt/V calculated using the end-dialysis blood urea was higher (mu 21%, SD 5) than when the more correct equilibrated value was used. The post-dialysis rebound indicates significant disequilibrium between the two pools at the end of dialysis. Dialysis efficiency may be substantially overestimated unless this is allowed for by using the rebounded post-dialysis blood urea when calculating Kt/V.