Increased levels of platelet associated IgG in patients with thrombocytopenia are not confined to any particular size class of platelets.

Patients with immune thrombocytopenia have an increased percentage of microthrombocytes/platelet fragments and megathrombocytes. It has been suggested that increased levels of platelet associated IgG (PA-IgG) found in these patients might be related to the presence of this abnormal platelet size distribution. In this study we used flow cytometry to investigate the distribution of PA-IgG within a population of platelets and, in particular, we examined the relationship between platelet size and PA-IgG determined simultaneously on individual platelets. Platelet samples from 10 normals and 31 thrombocytopenic patients were studied. PA-IgG was estimated using immunofluorescent FITC anti-IgG antibody. Binding of FITC anti-IgG to the platelets was quantitated in the flow cytometer as relative mean fluorescence (RMF) which was calibrated against values (in fg/plt of FITC anti-IgG) obtained by spectrofluorometry after solubilization of the platelets. A high correlation (r = 0.89) was found between flow cytometric RMF value and spectrofluorometric FITC anti-IgG values. The flow cytometric studies showed that platelet samples with abnormally elevated levels of FITC anti-IgG (greater than 1.7 fg/plt) not only have a higher percentage of platelets with elevated FITC anti-IgG, but that these platelets also have increased levels of FITC anti-IgG as compared to platelets from normal samples. Platelet size was measured by the amount of forward light scatter in the flow cytometer. A low but significant correlation (r = 0.33 +/- 0.12) was found between size (FALS) and fluorescent signals in samples with elevated FITC anti-IgG. The contribution of 10% of the smallest platelets by FALS and 10% of the largest platelets by FALS to the total levels of flow cytometer platelet fluorescence in these samples was only 4.4% and 19.4% respectively which was not higher than obtained with samples with normal levels of FITC anti-IgG. In conclusion, this study showed that increased levels of PA-IgG found among thrombocytopenic patients were not confined to any particular size class of platelets.