Pseudothrombocytopenia. In vitro studies on the underlying mechanism.

In a prospective 9 month study at a medical department with special interest in immunology and rheumatology, 10 patients with spuriously low thrombocyte counts - pseudothrombocytopenia - were registered. The phenomenon was observed when the counting of platelets was performed on a Thrombocounter C apparatus or by manual counting using K2EDTA stabilized blood since platelet agglutination was considerably less pronounced when solution citratis glucosi fortis (ACD) was used as the anticoagulant. Results of our in vitro studies indicate that the mechanisms contributing to pseudothrombocytopenia involve the interaction of circulating immune complexes with platelet membrane Fc receptors causing agglutination. Fab'2-fragments of isolated patient IgG did not react with normal donor thrombocytes. This finding argues against agglutination due to true antiplatelet antibody activity.