Apheresis without substitution fluid: devices, directions and limitations.

Significant economic and medical drawbacks attend the requirement for infusion of large quantities of exogenous substitution fluid during therapeutic plasma exchange. A variety of physicochemical processes are available for the fractionation of plasma (ultrafiltration, fractional precipitation, adsorption, electrophoresis, selective enzymatic degradation) and we review here their application to 'closed loop' apheresis, a format in which the patient's own plasma is returned after the pathogenic species are removed. The different separation processes vary significantly in their method of action, their specificity, and their status of development. Disposables and hardware for two specific protocols, cascade filtration and cryofiltration, are already commercially available and appropriate for routine clinical use in limited subsets of diseases considered treatable by therapeutic apheresis.