Impact of ultrafiltration on back-diffusion in hemodialyzer

Ultrafiltration of water from blood to dialysate decreases the rate of back-diffusion of solutes from dialysate to blood. Therefore, back-clearance (bK) of hemodialyzers may be expressed as bK = bK0--bTrQu, where bK0 is the diffusive back-clearance, bTr is the "back-"transmittance coefficient, and Qu is the net ultrafiltration rate. A formula for bK was derived from the one-dimensional theory of hemodialyzer, and bTr was described as a function of bK0 and the Staverman reflection coefficient. The transport parameters, bK0 and bTr, for creatinine and vitamin B12 were measured in two types of hemodialyzers with negligible back-filtration, using water solutions, and compared with the transport parameters, K0 and Tr, for the case of both diffusion and ultrafiltration from blood to dialysate. bK0 was in general equal to K0. bTr was not different from Tr for creatinine whereas bTr was lower than Tr for vitamin B12. Experimental values of bTr for vitamin B12 were in general agreement with theoretical predictions. However, experimental values of bTr for creatinine were lower than predicted values. We conclude that the impact of ultrafiltration on back-clearance for slowly diffusing solutes is weaker than on their clearance.