Platelet surface-bound IgG in patients with immune and nonimmune thrombocytopenia.

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. An analysis of the distribution of values in these two groups indicated that 90% of the patients with immune thrombocytopenia had greater than 800 molecules of IgG/platelet, whereas only 7% of patients with nonimmune thrombocytopenia exceeded this amount. The immune thrombocytopenia patients included 39 idiopathic, 14 secondary, and 14 drug-induced disorders, and they did not significantly differ in their distribution of values for platelet IgG. The nonimmune thrombocytopenic patients included 12 cases with a platelet destructive mechanism; their platelet-bound IgG was similar to that of the other nonimmune patients. Twenty-seven patients with treatment resistant immune thrombocytopenia were also studied they had 2,100 +/- 670 molecules of IgG/platelet. Their values were significantly greater than those of the nonimmune thrombocytopenic patients and not significantly different from those of immune thrombocytopenic group. 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.