Clinical applications of therapeutic apheresis

The vast majority of the renal indications for plasmaexchange are related to immunoglobulin removal.Immunoglobulins, especially IgG, have a relativelylong half-life. Thus in antibody-mediated disease, therecould be persistence of significant amounts of antibodyin the circulation despite cessation of antibody produc-tion. The aim of plasma exchange is to significantlyreduce circulating antibodies. Removal of the circulat-ing antibodies constitutes the rationale for using plas-mapheresis to treat antibody-associated glomeruloneph-ritis (GN). Although small molecular weight substancesare removed by plasma exchange, their large volumeof distribution and short half-lives make plasmaexchange an inefficient means of extracorporeal re-moval of these substances. For instance, some comple-ment proteins have a half-life of 2 days. If the goalwere to be to deplete plasma complement levels, virtu-ally daily plasma exchanges would be needed. Discon-tinuation of daily plasma exchange would be followedby rapid resurgence to normal complement titers.Hence the shorter the half-life of the molecule beingremoved, the more aggressive has to be the apheresisschedule.Plasma volume can be estimated using the followingformula:EPV ¼ 0:065 3 TBW 3 ½1 Hctwhere EPV is the estimated plasma volume, TBW isthe total body water, and Hct is hematocrit.The removal of large molecular weight substancesfrom the plasma compartment follows first-orderkinetics. Repetitive treatments should be spaced every24–48 h to allow for extravascular to intravascularequilibration.Apheresis has been used to treat several renal condi-tions including primary renal diseases as well as renalmanifestations of systemic conditions (Table I).