Urea kinetic and middle molecule approaches to assessing the adequacy of hemodialysis and CAPD.

The historical bases and implications of the urea kinetic and middle molecule approaches to assessing the adequacy of hemodialysis and continuous ambulatory peritoneal dialysis (CAPD) are reviewed in this paper. A critical reappraisal of the findings of the National Cooperative Dialysis Study (NCDS) demonstrates the importance of small solute removal on clinical outcomes of morbidity and mortality in the hemodialysis setting. The applicability of these results to CAPD is examined in the light of differences in transport characteristics of hemodialysis membranes and the peritoneal membrane. It is shown that small solute removal is important even in assessing CAPD adequacy and that past comparisons of hemodialysis and peritoneal membrane transport may have been flawed. The extrapolation of NCDS results to CAPD on the basis of the peak concentration hypothesis is presented along with a review of several longitudinal, cross-sectional, and multicenter studies of CAPD morbidity and mortality.