B2M and hydrops fetalis: This novel analysis suggests that changes in β2M concentration explain one third of the estimated benefit of minimally reused HF dialyzers, while the remainder is accounted for by direct effects which are not related to β2M. Thus, in spite of the theoretical predominance of the “middle molecule” hypothesis on dialysis technology research[62], a closer examination of HEMO suggests that removal of these uremic solutes may have only a modest effect on outcomes.