Simultaneous enzymo-immunologic assays of platelet associated IgG, IgM and C3. A useful tool in assessment of immune thrombocytopenias.

A solid phase enzymo-immunologic assay (EIA) has been developed to measure platelet associated (PA) IgG, IgM and C3. Washed platelets are mixed with a goat anti-IgG (IgM or C3) antiserum and then incubated in serum coated polystyrene plates. There is an inverse relationship between the level of PA IgG (IgM or C3) and the amount of goat antibodies binding to the IgG (IgM or C3) coating the plates. These goat antibodies are detected by addition of a phosphatase-labelled sheep anti-goat immunoglobulins antibody followed by a substrate giving a colour reaction. Using this technique, platelets from normal donors gave values of 2.04 +/- 1.10 fg IgG/platelet (mean +/- SD), 8.66 +/- 2.61 x 10(-16) g IgM/platelet and 5.67 +/- 2.63 x 10(-16) g C3/platelet. In 35 thrombocytopenic ITP patients we observed an increased level of PA IgG in 25, of PA IgM in 24 and of PA C3 in 23, and all of them had at least an increase of one of the 3 components. In 10 ITP patients in remission, 2 had an increased level of PA IgG while PA IgM and C3 values were normal in all. On the other hand, 2 non thrombocytopenic patients with systemic lupus erythematosus had an increased level of PA IgM. Compared to the standard antiglobulin consumption assay (ACA), the EIA was simpler to perform and more sensitive.