Detection of Platelet Autoantibodies by a Radioactive Anti-Immunoglobulin Test

Abstract. A direct and an indirect version of a platelet radioactive anti-immunoglobulin test (PRAT) has been developed. It is shown that this assay is sensitive, reproducible and quantitative for the demonstration of HLA as well as incomplete P1A1 antibodies. Furthermore, platelet autoantibodies were searched for in sera of 105 thrombocytopenic patients. Positive results were obtained in 15 out of 45 patients with chronic idiopathic thrombocytopenic purpura (ITP), in 1 of 17 cases with acute ITP and in 3 of 4 patients with ITP-like syndrome of systemic lupus erythematosus (SLE). The platelet autoantibodies mostly had low titers, reacted with autologous platelets and did not show any specificity. An elevated concentration of IgG on autologous platelets was detected by the direct PRAT in 11 of 18 patients with ITP and in 4 of 5 patients with thrombocytopenia and suspected SLE. 4 of the 7 negative ITP cases were in clinical remission or had platelet counts of over 50,000/μl. There appeared to be an inverse correlation between the concentration of surface-bound IgG and the platelet number in patients with ITP and SLE. It is concluded that the PRAT is a new and valuable tool for the detection of various types of platelet antibodies and therefore a true alternative to existing serological techniques in platelet immunology.