Blood
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We quantitated the amount of platelet surface-bound IgG using an 125I monoclonal anti-IgG assay in 149 patients with thrombocytopenia and 260 normal donors. The normal subjects had 122 +/- 5 molecules of IgG/platelet (mean +/- SE). Fifty-five patients with nonimmune thrombocytopenia had 338 +/- 37 molecules of IgG/platelet, whereas 67 patients with immune thrombocytopenia studied at the time of their initial evaluation had 4,120 +/- 494 molecules of IgG/platelet. ⋯ Their distribution of values was much broader, however, with 33% of patients having less than 800 molecules of IgG/platelet, suggesting possible alternate mechanisms in their thrombocytopenia. Thus, patients with immune thrombocytopenia have a high frequency of elevated IgG on the platelet surface which reflects the pathophysiology of this disorder. Quantitation of platelet-bound IgG provides a useful laboratory tool in the differential between immune and nonimmune thrombocytopenia.