Background: There is a concern that high haematocrit (Hct) levels will reduce the efficiency of dialysis treatments, particularly in post-dilution haemodiafiltration (HDF) where there is the potential for intense haemoconcentration within the dialyser. Methods: We measured serial Hct and performed serial clearance measurements for urea, phosphate, β2-microglobulin and myoglobin in 12 patients with Hct >35% on high-flux haemodialysis (HFHD) or HDF. We assessed whether changes in the intra-dialyser Hct influenced solute clearance and whether there were differences between the two modalities. Results: Hct rose significantly in all treatments studied. Convective and total solute clearances were higher in the HDF group when compared to the HFHD group. Phosphate clearance in HFHD fell towards the end of dialysis when the Hct was highest but no differences were detected for the other solutes. Conclusion: Despite marked haemoconcentration, the impact on solute clearance across the range of molecular size studied is small. These findings are reassuring in the current era of widespread erythropoietin use.

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