Apheresis, exchange, adsorption and filtration of plasma: four approaches to the removal of undesirable circulating substances.

Plasmapheresis and plasma exchange have been widely used in the treatment of a variety of conditions, not always with a clear rationale. The most favorable results have been observed in the hyperviscosity syndrome and in a group of antibody-mediated diseases which includes post-infectious cold agglutinin disease, red cell aplasia, post-transfusion purpura, Goodpasture's syndrome, hemophilic patients with anti-factor VIII antibodies, and also in thrombotic thrombocytopenic purpura. In contrast, disappointing or conflicting results have been obtained in immune complex diseases, myasthenia gravis, cryoglobulinemia, etc. It seems likely that at least some of the difficulties may arise from the non-specific removal of a variety of substances as well as for our lack of understanding of immunological feed-back mechanisms which are disturbed by plasmapheresis and related techniques. Future developments are likely to be centered in the development of more specific approaches as based on filtration, affinity chromatography and adsorption to antibodies or other substrates. These approaches appear promising for the removal of "blocking factors" in patients with cancer, lipoproteins in patients with hypercholesterolemia, immune complexes, and toxic compounds. A cautiously optimistic approach to such new developments and their testing in animal models and in carefully controlled patient trials are essential. The principles on which these therapeutic approaches lie are valid, and the skepticism that surrounds them may be underserved, since it was largely the result of an indiscriminate use of non-selective procedures.