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Since therapeutic conditions, especially the amount of dialysate, are usually limited in continuous renal replacement therapy (CRRT), selecting an appropriate membrane is more crucial than that in chronic hemodialysis. Under such circumstances, the use of a membrane with adsorption is expected to remove a larger amount of target substances in CRRT. Five commercial dialyzers were investigated to demonstrate the importance of membrane characteristics. The adsorptive characteristics of polymethylmethacrylate (PMMA) membrane were relatively low for cytochrome c (MW 12,400), very strong for alpha-chymotrypsinogen A (MW 25,000) and relatively strong for albumin (MW 66,000), which may be understood that the adsorption in PMMA has the optimal molecular size. On the other hand, polyacrylonitrile showed relatively low affinity and polysulfone showed essentially no affinity to these protein molecules. Time- and concentration-dependent characteristics of clearance for these proteins were also demonstrated in PMMA. Then we concluded that adsorption found in PMMA may be due to the occlusion of protein molecules into pores of entirely dense membrane. Selecting membrane materials is, therefore, important not only in removing inflammatory cytokines but also in considering the loss of albumin in clinical treatments because even albumin can be adsorbed by the membrane used in blood purification therapies.

In a clinical situation, since membrane fouling often causes the reduction of solute removal performance of the dialyzer, it is necessary to evaluate the performance of the dialyzer, considering the effects of fouling even in aqueous in vitro experiments that are useful for the better design of dialyzers. We replicated the membrane fouling by immobilizing albumin on the membrane in a dialyzer using glutaraldehyde as a stabilizer. The modules of various membrane surface areas with and without replication of the fouling were used for performance evaluation of solute (creatinine, vitamin B12, and inulin) removal in dialysis experiments in vitro. Clearances for these solutes in the modules with fouling were lower than those without fouling. Furthermore, the smaller the surface area, the larger the fouling effect was observed in all solutes. Calculated pressure distribution in a module by using a mathematical model showed that the solute removal performance might be greatly affected by the rate of internal filtration that enhances the solute removal, especially for larger solutes. The increase in the rate of internal filtration should contribute to improving the solute removal performance of the dialyzer, with a higher effect in modules with a larger membrane surface area.

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