Impact of solute exchange between erythrocytes and plasma on hemodialyzer clearance

The standard theory of mass transport in dialyzer for water solutions was extended for solutes distributed in both plasma (PW) and erythrocyte intracellular (EW) water. Blood flow was divided into two separate flows of PW and EW with the diffusive exchange of solutes across cellular membrane (CM). Diffusive permeability of CM for urea and creatinine were assumed according to literature data. Computer simulations based on partial differential equations demonstrated that urea diffuses fast across CM and can be approximately considered as distributed uniformly in both blood flow components. In contrast, creatinine can be considered as distributed only in PW flow during the passage along the dialyzer. Therefore, the traditional formula for dialyzer clearance can be applied for urea and creatinine with the adjustment of their effective “blood” flow, but not for solutes with intermediate molecular mass. In vivo clearances of urea and creatinine were, as expected, lower than the respective theoretical predictions based of the diffusive permeability, P, times membrane surface area, A, parameters, PA, for dialyzer membrane, estimated for water solutions, by 33.6±10.9% for creatinine and 10.8±9.4% for urea. The estimated in vivo PAs were for creatinine 65.4±26.0% and for urea 32.0±10.9% lower than in vitro values provided by manufacturers. The much higher drop in clinical clearance/PA for creatinine than for urea suggests that the exchange of creatinine between plasma and dialysis fluid needs to be adjusted for the reduction of the dialyzer membrane surface area, which is effectively available for creatinine, caused by the presence of erythrocytes.