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-cl earance (bK) of hemodialyzers may be expressed as bK = bK(0) - bTrQ(u) , where bK(0) is the diffusive back-clearance, bTr is the ''back-''tra nsmittance coefficient, and Qu is the net ultrafiltration rate. A form ula for bK was derived from the one-dimensional theory of hemodialyzer , and bTr was described as a function of bK(0) and the Staverman refle ction coefficient. The transport parameters, bK(0) and bTr, for creati nine and vitamin B-12 were measured in two types of hemodialyzers with negligible back-filtration, using water solutions, and compared with the transport parameters, K-0 and Tr, for the case of both diffusion a nd ultrafiltration from blood to dialysate. bK(0) was in general equal to K-0. bTr was not different from Tr for creatinine whereas bTr was lower than Tr for vitamin B-12. Experimental values of bTr for vitamin B-12 were in general agreement with theoretical predictions. However, experimental values of bTr for creatinine were lower than predicted v alues. We conclude that the impact of ultrafiltration on back-clearanc e for slowly diffusing solutes is weaker than on their clearance.