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Adsorption in hemodialysis. The use of sorbents in different blood purification techniques is reviewed. The sorbents used in these therapies are divided into two groups: (1) Adsorption occurs fundamentally because of the hydrophobic properties of the sorbents. In this group, the sorbents used in different dialysis techniques are charcoal and nonionic macroporous resins. (2) Adsorption occurs by chemical affinity, such as ion exchange resins and chemisorbents. Sorbents were initially used in hemoperfusion, which caused many adverse events; later, with the use of coated charcoal, these undesired effects decreased or disappeared, but the adsorptive properties, water control, and acid-base balance still created problems. For these reasons, the use of sorbents in the treatment of chronic renal failure was almost totally discontinued. Little by little, interest in these substances has reappeared, and at present, they have been used in combination with other blood purification techniques such as hemodialysis, hemofiltration, peritoneal dialysis, and finally, hemodiafiltration. Within the various hemodiafiltration techniques, paired filtration dialysis-charcoal is being used to regenerate the ultrafiltrate, which is used as the replacement fluid. Charcoal regenerates the ultrafiltrate and transforms it into a physiological solution with a normal electrolyte composition, calcium, bicarbonate, and glucose, having eliminated the majority of both middle and large molecule uremic toxins. If regeneration is done properly, this replacement fluid is bacteria and endotoxin free. Studies currently are underway on the adsorption of different inflammatory substances in the ultrafiltrate, which could lead to improvement in the biocompatibility of the system.