Theoretical basis of pathogenic substance removal during plasmapheresis.

This article explains theoretical considerations of prescribing plasmapheresis. It is important to consider several factors in prescribing apheresis: (i) the properties of pathogenic substances, such as molecular weight, distribution volume, compartment, and production rate; (ii) therapeutic conditions such as the processed plasma volume and the frequency of processing; and (iii) the patient's condition, such as active bleeding or infection. A substance's molecular weight determines whether it can be removed using a particular membrane filter. Substances with a small distribution volume and low production rate are removed. Consequently, the processed volume per total plasma volume is related to the single-session efficacy. Nevertheless, even frequent therapy cannot reduce the total pool of a substance within the body if movement from outside the vessels is slow. Active bleeding or infection might be exacerbated by modalities other than simple plasma exchange with fresh plasma supplementation because such therapies cannot replenish substances other than albumin.