In this report we describe the use of an 125I-Staphylococcal protein A (SPA) assay to measure platelet-bound IgG in the evaluation of 62 thrombocytopenic patients. Platelets from 150 normal subjects were found to bind 146 +/- 112 molecules of SPA per platelet (mean +/- 2 SD). Nineteen of 20 patients with untreated immune thrombocytopenia had platelet IgG values above this range, with 15 of 20 having values above 1,000 molecules of SPA per platelet. Patients with immune thrombocytopenic purpura by clinical criteria, but who had failed conventional therapy (corticosteroids or splenectomy), had a wide range of platelet IgG levels: 4 of 20 had normal values, 6 of 20 had minimally elevated levels in the range seen with nonimmune thrombocytopenia, and 10 of 20 had much higher values. Fifteen patients with thrombocytopenia of apparent nonimmune origin and 7 others with chronic stable thrombocytopenia of unknown etiology were found to have platelet IgG levels within or only slightly above the normal range. Because of its simplicity, accuracy, and clinical correlation, the 125I- SPA assay provides an important new approach for studying platelet IgG in thrombocytopenic states. The data obtained with this technique are similar to those found in immune hemolytic anemia and suggest that the platelet-bound IgG so measured has pathophysiologic relevance in immune thrombocytopenic purpura.

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