Effect of increasing dialysate flow rate on diffusive mass transfer of urea, phosphate and β2-microglobulin during clinical haemodialysis

Background. Diffusive clearance depends on blood and dialysate flow rates and the overall mass transfer area coefficient (KoA) of the dialyzer. Although KoA should be constant for a given dialyzer, urea KoA has been reported to vary with dialysate flow rate possibly because of improvements in flow distribution. This study examined the dependence of KoA for urea, phosphate and β2-microglobulin on dialysate flow rate in dialyzers containing undulating fibers to promote flow distribution and two different fiber packing densities. Methods. Twelve stable haemodialysis patients underwent dialysis with four different dialyzers, each used with a blood flow rate of 400 mL/min and dialysate flow rates of 350, 500 and 800 mL/min. Clearances of urea, phosphate and β2-microglobulin were measured and KoA values calculated. Results. Clearances of urea and phosphate, but not β2-microglobulin, increased significantly with increasing dialysate flow rate. However, increasing dialysate flow rate had no significant effect on KoA or Ko for any of the three solutes examined, although Ko for urea and phosphate increased significantly as the average flow velocity in the dialysate compartment increased. Conclusions. For dialyzers with features that promote good dialysate flow distribution, increasing dialysate flow rate beyond 600 mL/min at a blood flow rate of 400 mL/min is likely to have only a modest impact on dialyzer performance, limited to the theoretical increase predicted for a constant KoA.