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Anna Lindgren,1,2 Erik Fjellstedt,2,3 Anders Christensson2,3 1Department of Nephrology, Skåne University Hospital, Lund, Sweden; 2Department of Clinical Sciences Malmö, Lund University, Malmö, Sweden; 3Department of Nephrology, Skåne University Hospital, Malmö, SwedenCorrespondence: Anna LindgrenDepartment of Clinical Sciences Malmö, Lund University, SUS Malmö, Jan Waldenströms Gata 15, Malmö SE-205 02, SwedenTel +46-46-171698Fax +46-40-337052Email Anna.Lindgren@med.lu.sePurpose: Conventional hemodialysis (HD) treatment has an acceptable removal of small uremic molecules, but so-called “middle molecules” in the range of 0.5– 60 kDa are poorly cleared with HD compared to a native kidney, which may contribute to morbidity in the dialysis population. Hemodiafiltration (HDF) has a better removal of middle molecules compared to HD but is technically demanding and requires well-functioning dialysis access. The newly introduced medium cutoff (MCO) filters have been developed to enhance middle molecule clearance in HD-mode. The aim of this study was to compare reduction ratios (RRs) of molecules with different molecular weights (0.06– 150 kDa) during dialysis with MCO dialyzer (used in HD-mode) compared to online-hemodiafiltration (ol-HDF) treatment with a conventional high-flux dialyzer.Patients and Methods: This is a prospective controlled single-center cross-over study, including 16 patients in Malmö, Sweden. All patients had ongoing post-dilution ol-HDF treatment before the study. The study compared reduction ratios of small-, middle-, and large-sized molecules during a single 4h dialysis treatment with post-dilution ol-HDF (Polyflux 210H) to a 4h dialysis treatment with MCO dialyzer (Theranova 500) in HD-mode. Between treatments, the patients had a washout period of at least two weeks of their ordinary HDF treatment to reach their ordinary steady state.Results: ol-HDF had significantly higher RR for cystatin C (13 kDa), compared to MCO (RR 68.1 vs 65.8, p=0.003), during a 4h dialysis treatment (mean convection volume of 24.5 L for HDF, and mean Qb of 324 mL/min for HDF and 323 mL/min for MCO). There was no significant difference in the RR for other middle molecules, or for smaller or larger molecules.Conclusion: Overall, the RRs were comparable for ol-HDF and MCO-HD. There was a slightly higher RR of cystatin C (a small middle molecule) for HDF compared to MCO but no difference in other measured molecules.Keywords: chronic hemodialysis, dialysis, hemodiafiltration, hemodialysis, MCO, uremic toxins