The use of 125I labelled staphylococcal protein A in the diagnosis of autoimmune thrombocytopenic purpura and other immune mediated thrombocytopenias.

Platelet-associated immunoglobulin (IgG) has been measured directly on platelets from normal and thrombocytopenic subjects by a modified radioactive Coombs technique. Platelet-associated IgG was detected using 125I-staphylococcal protein A after which platelets were separated from unbound 125I-protein A by centrifugation through oil. Elevated values of platelet-associated IgG were observed in 17 of 19 patients with autoimmune thrombocytopenic purpura. All patients with thrombocytopenia associated with systemic lupus erythematosus or rheumatoid arthritis showed elevated platelet-associated IgG, while normal values were found in diverse thrombocytopenias which were non-immunological in origin. Measurement of anti-platelet antibody levels by an indirect test was less reliable in diagnosing autoimmune thrombocytopenic purpura, since only 7 of the 19 patients gave elevated values. However, the indirect test was reliable in the diagnosis of quinine-dependent immune thrombocytopenia; all 6 patients with suspected quinine purpura gave strongly positive indirect tests when 1 mM quinine was added to the patient's plasma. The use of 125I-protein A to detect platelet-associated IgG provides a rapid and simple technique for the reliable diagnosis of immune thrombocytopenia.