<qa>  What is the possibility of bias by confounding by indication?Thus far, no definite data on the effects of HDF on CV parameters andor clinical end-points are available. Promising data include a reduction of left ventricular mass index LVMi after one year of treatment with acetate free bio-filtration AFB. Furthermore, relatively high survival rates were reported in a single center non-experimental study on patients who were treated with HDF, if compared to the EDTA registry data on HD-treated patients. Yet, these data are of observational nature, with the possibility of being biased by confounding by indication. 