Flow cytometric detection of circulating activated platelets in primary antiphospholipid syndrome. Correlation with thrombocytopenia and anticardiolipin antibodies.

Platelet activation has been suggested to play a crucial role in the pathogenesis of haemostatic disorders in antiphospholipid syndrome (APS). In 16 patients with primary APS (PAPS) we investigated by flow cytometry the presence of circulating activated platelets as defined by the surface expression of activation-dependent glycoproteins CD62 and CD63. In addition, the relationships among activated platelets, thrombocytopenia, antiphospholipid antibodies (aPL) and platelet associated IgG (PalgG) were evaluated. Compared to normal subjects CD62, but not CD63 expression, was found significantly increased in patients. All thrombocytopenic subjects showed a percentage of CD62 expressing platelets above the cut off. In thrombocytopenics a significantly increased percentage of CD62 and higher levels of aCL IgG were found compared to PAPS patients with normal platelet count. No correlation was found between activated platelets and both lupus anticoagulant antibodies and PalgG. Our data demonstrate that circulating activated platelets are detectable by flow cytometry in the majority of PAPS patients and suggest the existence of a relationship among activated platelets, thrombocytopenia and aPL levels.