EDTA dependent pseudothrombocytopenia caused by antibodies against the cytoadhesive receptor of platelet gpIIB-IIIA

Abstract

Aims-To clarify the mechanisms involved in the development of EDTA dependent pseudothrombocytopenia, particularly the platelet receptors. Methods -Platelets were measured in 33 patients with pseudothrombocytopenia, using different anticoagulants to coliect blood samples (direct test). The results were compared with the counts obtained by adding patients' serum or immunoglobulins to normal blood samples (indirect test). The role of platelet function was explored using ASA, PGEl, and apyrase as platelet inhibitors. The contribution of platelet receptorls was investigated using antigens to gplb-lX and gpIlb-IIIa monoclonal antibodies. Immunoglobulin class was estimated by the ability of IgG, IgA, and IgM antibodies to prevent platelet clumping. Results-Agglutinating antibodies were IgA in 400/o, IgG in 30%, and IgM in 10%/o of patients studied. Both patients' serum and immunoglobulins induced platelet clumping in normal samples anticoagulated with EDTA (indirect test). This was prevented by incubation ofblood samples at 37°C and almost completely inhibited by the platelet inhibitors ASA, PGE1, and apyrase. Pseudothrombocytopenia was also entirely prevented by an antigen to gpIlb-Ma monoclonal antibody that recognises fibrinogen and the von Willebrand factor binding site. Pseudothrombocytopenia was almost completely abolished after the addition of RGD peptide, the recognition sequence of cytoadhesive proteins. Conclusions-These findings suggest that EDTA dependent pseudothrombocytopenia is caused by agglutinating antibodies that recognise cytoadhesive receptors on platelet gpllb-Ma and that an efficient platelet metabolism is required.