Immune thrombocytopenia. Use of a Coombs antiglobulin test to detect IgG and C3 on platelets.

We applied a radiolabeled Coombs antiglobulin test to the diagnosis and management of immune thrombocytopenia in adults and children. This assay substantiated that the majority of patients with idiopathic thrombocytopenic purpura have increased levels of IgG on their platelets. Platelet-associated C3 was elevated in a subset of these patients, some of whom had normal levels oet-associated IgG, thus suggesting a role for C3 in the pathogenesis of the thrombocytopenia. The assay enabled us to anticipate a change in clinical status, establish the presence of immunologic remission after splenectomy and propose several mechanisms by which corticosteroids act. Platelets from a patient with the post-transfusion-purpura syndrome also carried increased IgG, indicating a role for IgG antibody or IgG-containing immune complexes in the destruction of host platelets in this disease. The radiolabeled Coombs test provides a general means to help diagnose, manage and study immune platelet disorders.