Background: Current methods of renal replacement therapy lead only to an insignificant removal of larger, potentially toxic, substances, which are excreted by healthy kidneys. On-line preparation of substituate from dialysate and the use of high-flux membranes allow substantial convective removal of such substances. A modified on-line haemodiafiltration method with the use of a large membrane surface and a high convective part was chosen to test whether the elimination of larger substances, such as low-molecular-mass proteins, has a clinical impact.
Methods: In a prospective, controlled study over 24 months, 44 unselected chronic dialysis patients were randomized to undergo either low-flux haemodialysis (HD; n = 21) or haemodiafiltration (HDF; n = 23). To eliminate confounding factors, low-molecular efficacy was matched (Kt/V 1.8), and the same membrane material (polysulfone), ultrapure dialysate and the same treatment duration (4.5 h) were applied to each group.
Results: Morbidity, mortality, blood pressure, dialysis-associated hypotensive episodes, haematocrit and erythropoietin dose did not differ between the groups. The same was true for body weight and, accordingly, bioimpedance values, clinical hydration score, skinfold thickness, plasma albumin, prealbumin and transferrin. beta2-Microglobulin in the plasma did not change in the HD group and varied between 32 and 43 mg/l throughout the 2 years. In HDF, beta2 microglobulin decreased from similar values to 18 mg/l predialysis (P<0.01) in the first 6 months of HDF treatment and then remained constant during the remaining 18 months.
Conclusion: In the absence of any clinical marker of uraemic toxicity the removal of larger molecules over the time-span of 2 years during HDF had no clinical implication compared with extremely (and for routine practice unrealistically) well-dialysed patients with low-flux HD. In the absence of any side-effects of on-line HDF and supposing that plasma beta2-microglobulin is a marker of morbidity, on-line HDF ensures an excellent dialysis quality which apparently takes time to translate into measurable clinical sequelae.