Hemodiafiltration--a new treatment option for hyperphosphatemia in hemodialysis patients.

Background: Hemodiafiltration is used to increase the convective transport and thereby the elimination of small and middle molecules, mainly β 2 -microglobulin (β 2 -M) across the dialysis membranes. There is little information concerning urea, creatinine, β 2 -M and principally phosphate kinetics during hemodiafiltration in vivo. In this prospective study, we evaluated the transmembrane solute mass removal (TSR) and clearance (Kd) of urea, creatinine and phosphate as well as serum β 2 -M reduction rate (β 2 -MRR) and collected β 2 -M in dialysate plus ultrafiltrate during high-flux hemodialysis (HD) and post-dilutional hemodiafiltration (HDF). Patients and methods: 16 patients were studied using a polysulfone capillary filter (1.6 m 2 surface area, 40 μm fiber internal diameter and 200 μm, wall thickness) during 2 one-week periods: first week HD 1.6 m 2 and second week HDF 1.6 m 2 . Treatment time was 4 hours, blood flow rate 300 ml/min with constant dialysate and ultrafiltration rates for HD and HDF periods. TSR, Kd, β 2 -MRR and β 2 -M collection were assessed during the mid-week treatment. In a second part of the study, we repeated the same protocol using a second high-flux polysulfone capillary filter (2.4 m 2 surface area, 30 μm fiber diameter and 150 μm wall thickness). Results: TSR and Kd of urea and creatinine were not improved by HDF, however, HDF increased TSR and Kd of phosphate. Phosphate clearance rose from 120 (HD 1.6 m 2 ) to 159 (HDF 1.6 m 2 ) (p < 0.005) and from 146 (HD 2.4 m 2 ) to 206 (HDF 2.4 m 2 ) (p < 0.005) ml/min. β 2 -MRR increased from 64.1 ± 8.6 to 77.7 ± 8.2% (p < 0.005) and from 75.0 ± 5.1 to 82.9 ± 8.5% (p < 0.005) during HDF 1.6 m 2 and HDF 2.4 m 2 , respectively. Collected β 2 -M remained unchanged. This discrepancy seems to be due to an enhanced β 2 -M adsorption to the polysulfone membrane during HDF. Conclusion: Our results provide a strong evidence that HDF has no advantage over HD with respect to urea and creatinine removal in vivo. However, HDF did improve the elimination of phosphate and should be considered as an additional treatment option for hyperphosphatemia in dialysis patients. HDF improves significantly the elimination of β 2 -M.