What are we expecting to learn from the MPO study?

High-flux membranes represented a major improvement in dialysis technique, but evidences supporting their clinical superiority over conventional low-flux dialysis are still inconclusive. Although several studies, most of which were observational, showed an association between high-flux dialysis and lower morbidity and mortality, the Hemodialysis (HEMO) study, the first large-scale randomized clinical trial specifically aimed at testing the effect of membrane permeability on patients' outcome, failed to demonstrate a statistical significant benefit of high-flux membranes on all-cause mortality. Although disappointing, these results should however be interpreted in light of some important limitations of the HEMO study, first of all the inclusion of both incident and prevalent hemodialysis patients, the exclusion of sicker patients and the allowance of dialyzer reuse. In this context, much is expected from the Membrane Permeability Outcome (MPO) study, a randomized clinical trial investigating the effect of high-flux membranes in a large population of incident hemodialysis patients across Europe. Inclusion of only incident patients, absence of severe exclusion criteria and no dialyzer reuse are all distinguishing features of this study. Analyses of the baseline data of the MPO study confirm the high burden of cardiovascular disease among incident dialysis patients, although comparison with the Dialysis Outcomes and Practice Patterns Study data provides further evidence of a positive selection of patients in clinical trials.