Differences in solute removal by two high-flux membranes of nominally similar synthetic polymers

Background. Membranes fabricated from nominally similar polymers may be markedly different in chemical composition, morphology and geometry. To examine the relative importance of these factors to dialyzer performance, the removal of small and large uraemic toxins was determined for dialyzers containing 'polysulfone' membranes of different composition and morphology, with and without fibre undulations. Methods. Total removal and instantaneous clearances of urea, phosphorus, beta(2)-microglobulin, leptin, angiogenin, complement factor D and immunoglobulin kappa light chain were determined in randomized cross-over studies. Total solute removal was assessed from the pre- to post-dialysis change in plasma concentration and the total amount of solute recovered in the dialysate. Trapping of solute at the membrane was determined as the difference between solute lost from plasma water and solute recovered in the dialysate. Results. Total removal of urea and phosphorus was independent of the membrane composition and structure. Large molecule removal differed significantly between the two membranes, particularly for beta(2)-microglobulin. The importance of trapping at the membrane as a mechanism of beta(2)-microglobulin removal also differed significantly between the two membranes, with trapping being less important for the membrane with the greatest beta(2)-microglobulin removal. As molecular size increased, the contribution of trapping at the membrane to solute removal increased and the difference between the two membranes decreased. Conclusions. High-flux membranes fabricated from nominally similar polymers may differ significantly in their ability to remove low molecular weight protein uraemic toxins.