Characterization of antibodies directed against platelet cryptantigens detected during the immunological study of 356 consecutive patients with presumed autoimmune thrombocytopenia (AITP).

Cryptic antigens are detected by antibodies present in a wide spectrum of patients with or without thrombocytopenia, and even in healthy individuals. They are produced for unknown reasons and do not react with antigens of native platelets, but only with altered platelets. Cryptantigen antibodies may not only result in spuriously low platelet counts, but also in 'falsely' positive tests for platelet antibodies. We report our experience in the characterization of the different types of antibodies directed against cryptantigens of platelets: EDTA-dependent antibodies, PFA-dependent antibodies, EDTA-PFA-dependent antibodies and cold agglutinins. These antibodies were detected in the course of the serological study of 37 patients from a group of 356 (10%) whose blood was sent to our laboratory for platelet antibody testing. Pseudothrombocytopenia was diagnosed in 24 cases. Twenty-one of these showed EDTA-dependent or EDTA-PFA-dependent platelet agglutination and three were due to the presence of cold agglutinins. In 13 patients the thrombocytopenia was genuine. Eleven of these presented EDTA-dependent or EDTA-PFA-dependent antibodies in their serum and in the two remaining cases PFA-dependent antibodies were found. Cryptantigen antibodies were also detected in 9 out of 228 (4%) blood donors who were used as healthy controls in the platelet immunofluorescence test. In the light of the results obtained we put forward some guidelines to detect the presence of these antibodies and establish an accurate serological and clinical diagnosis of the autoimmune thrombocytopenias.