Platelet‐Associated IgG in Thrombocytopenia: A Comparison of Two Techniques

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Abstract
The results obtained in the analysis of 130 thrombocytopenic patients with a radioimmunoassay (RIA) for platelet‐associated IgG (PA‐IgG) and the platelet suspension immunofluorescence test (PIFT) were compared. The RIA was positive in 33 of 41 (82.9%) patients with idiopathic thrombocytopenia (ITP) and in 51 of 79 (64.4%) patients with secondary thrombocytopenia (STP). The PIFT was positive in 37 of the 41 (90.2%) ITP patients and in 57 of the 79 (72.2%) STP patients. Sensitivity and specificity for the diagnosis of ITP of both tests were comparable: 82.9 and 40.9% for the PA‐IgG(RIA) and 90.2 and 36.7% for the PIFT. A significant positive correlation was observed between the mean amount of PA‐IgG measured and the height of PIFT scores with anti‐IgG. Of 38 discrepancies between PA‐IgG(RIA) and PIFT with anti‐IgG, 15 were due to borderline results, 17 were associated with abnormal platelet‐size distribution and 20 were associated with occurrence of IgM antibodies. These results suggest influences of platelet fragments and/or aggregates on accurate measurement of PA‐IgG. Both fragments and aggregates escape from accurate platelet counting, while their contribution to the total IgG content remains. Therefore, a falsely elevated PA‐IgG (RIA) may be measured.