Alternative descriptions of combined diffusive and convective mass transport in hemodialyzer.

Two alternative versions of the mathematical description of the combined diffusive and convective membrane transport in hemodialyzers were compared using the one-dimensional theory of hemodialyzers. The first version is based on the assumption of homogeneity of the membrane. The second version is a widely used "ad hoc" formulation, which can be interpreted as a description of the membrane as tighter at the dialysate side than at the blood side. Theoretical predictions of the increase of dialyzer clearance caused by ultrafiltration, as assessed by transmittance coefficient, were compared to experimental data about transport of small solutes (urea, creatinine, and sodium) as well as middle molecules (vitamin B12) in three types of hollow-fiber hemodialyzer. For one type of dialyzer, the theory assuming the homogeneous membrane yielded the correct predictions for the small solutes. For two other types of dialyzer, the alternative version of the theory was adequate. For vitamin B12, the experimental values of transmittance coefficient were between the values predicted by the two versions of the theory for all three types of hemodialyzer. Thus, the two versions should be considered as a possible adequate description of solute transport in hemodialyzers.