Comparison of the direct platelet immunofluorescence test (direct PIFT) with a modified direct monoclonal antibody-specific immobilization of platelet antigens (direct MAIPA) in detection of platelet-associated IgG.

Glycoprotein (GP)-specific platelet-associated IgG (PA-IgG) may be demonstrable in autoimmune-mediated thrombocytopenia. We studied 159 consecutive patients with histories of thrombocytopenia by a modified direct monoclonal antibody-specific immobilization of platelet antigens (direct MAIPA) assay, which immobilizes GP IIb/ IIIa, GP Ib/IX and GP Ia/IIa simultaneously. This modification requires smaller quantities of platelets than standard measurements performed separately. PA-IgG was present in 84/159 (53%) patients, as shown by the direct platelet immunofluorescence test (PIFT) with flow cytometry as a reference. PA-IgG against GP IIb/IIIa and/or GP Ib/IX and/or GP Ia/IIa was noted in 46 patients (29%), of whom 93% (43/46)-were also PA-IgG positive. The amount of PA-IgG detected by PIFT correlated directly with that detected by direct MAIPA (r = 0.71; P < 0.001). Only three patients 12548 with negative direct PIFT had GP-specific PA-IgG. GPV-specific PA-IgG was detected in 13 (10%) of the 125 patients, in whom further studies could be performed. In the subgroup of patients with GP-specific PA-IgG, the median fluorescence intensities of direct PIFT were higher than in patients with no GP-specific PA-IgG (P < 0.001). Direct PIFT and direct MAIPA divided the patients into asymmetric subgroups. However, the relative roles of these tests in the diagnosis of autoimmune-mediated thrombocytopenia await further studies.