Posttransfusion purpura. Report of a case with anti-P1A1 masked by HLA antibodies.

A case of posttransfusion purpura is reported in which the laboratory determination of the specificity of the causative antibody was initially confused by the presence in the patient's serum of HLA-directed antibodies. The patient was a multiparous 65-year-old woman with a previous history of blood transfusion. She developed the typical clinical features of posttransfusion purpura 8 days following the transfusion of 6 units of packed red cells. The patient was P1A1 negative, and her serum reacted strongly in a radiolabeled monoclonal antibody assay with both P1A1-positive and -negative platelets. Radioimmunoprecipitation demonstrated that the patient's serum contained antibodies both of anti-P1A1 and anti-HLA specificity. Western blotting of normal platelets incubated with the patient's serum verified that the anti-P1A1 antibody was directed against platelet membrane glycoprotein III.