Large Middle Molecule and Albumin Removal: Why Should We Not Rest on Our Laurels?

Large middle molecules (LMM) are an important subclass of uremic toxins. Many of them have been linked with poor outcomes in hemodialysis (HD) patients. The onset of high-flux membranes and convective techniques allowed to dramatically improve their clearance but without a clear and undebatable reduction of mortality in HD patients. Despite the real effect on the removal of selected toxins, little is known about the influence of modern HD techniques on the global removal of uremic toxins. Mostly explained by a lack of knowledge and selective assays, LMM removal is not evaluated appropriately. The development of highly sensitive and widespread detection techniques such as mass spectrometry could increase our knowledge about the real state of their removal in HD. Nevertheless, these techniques remain cost effective and are difficult to handle. On the contrary, the improvement of LMM removal raises the question of a tolerable albumin removal. Indeed, increasing membrane permeability can significantly increase LMM removal accompanied by a higher albumin loss. However, in chronic kidney disease and particularly in HD patients, albumin can be modified, and it subsequently exerts detrimental effects. This could be avoided by the clearance of the modified forms in HD, but future efforts should be done to investigate the real impact of their removal.