A Comparison of Two Assays for Platelet Antibodies

The optimal method for assaying platelet bound antibody is uncertain. We have compared a fluorescent assay (FA) with a (quantitative) antiglobulin consumption assay for platelet associated IgG (PAIgG) in normals and thrombocytopenics with ITP, SLE, non-immune and drug-induced thrombocytopenia. Forty-eight of 49 hospitalized and healthy controls had normal PAIgG levels by the antiglobulin consumption assay (2.6 ± 0.2 fg IgG/platetet, ± SE, normal 0-5 fg) and the FA was negative in 41. The PAIgG level was elevated (20.0 ± 1.9 fq IgG/platdet) in 42 of 45 determinations on ITP patients. The FA was positive in 21. Positivity in the FA test did not relate closely to PAIgG level. The PAlqG was elevated (29.0 - 7.3) in 14 of 15 assays in thrombocytopenic SLE patients. The FA was positive in 3. The PAIgG level was elevated in all 9 patients with drug-induced thrombocytopenia (42.4 ± 23.2) without addition of the drug to the test system. The FA was positive in 5. In 2 of 12 patients with non-immune thrombocytopenia the PAIgG level was slightly elevated (both patients had multiple myeloma) and the FA was positive in 5. The results suggest that the quantitative antiglobulin consumption assay is more sensitive than the fluorescent assay in the diagnosis of immune mediated throinbocytopenia. The significance of the lack of correlation between positivity in the fluorescence test and the PAIgG levels found in patients with ITP is uncertain.