Enzyme-linked immunosorbent assay (ELISA) for direct quantification of surface-bound platelet immunoglobulins

Surface-bound platelet IgG and IgM were measured by an enzyme-linked immunosorbent assay (ELISA) using washed platelets and commercially available alkaline phosphatase anti-human immunoglobulins (Fc-specific). With this technique platelets from normal donors had small amounts of platelet-bound IgG ranging from 0.00 to 0.16 A405 (absorbance at 405 nm wavelength) (10(7) platelets)-1 (0 to 124 ng) and of platelet-bound IgM ranging from 0.00 to 0.05 A405 (10(7) platelets)-1. Eight out of 10 (80%) thrombocytopenic patients with idiopathic autoimmune thrombocytopenic purpura (IATP) had values of both IgG and IgM exceeding the normal range. In addition, one patient (8%) had platelet-bound IgM only. An inverse relationship was demonstrated in patients with IATP between the blood platelet count and the amount of both IgG and IgM. Increased values were also demonstrated in patients with SLE and patients with monoclonal hypergammaglobulinaemia. The direct ELISA is a useful and reproducible technique for platelet-bound IgG and IgM, which requires standard laboratory equipment only.