PLATELET ANTIBODIES IN IDIOPATHIC THROMBOCYTOPENIC PURPURA

Abstract

An immunofluorescence (IF) technique for detection of platelet antibodies was applied to idiopathic thrombocytopenic purpura (ITP). Serum platelet antibodies were found in 13 of 22 patients (59%) with active disease, but in only 4 out of 15 (27%) with attained remission. Direct tests for platelet-associated Ig[immunoglobulin]G were positive in 36 and 44% of these patients, respectively. In 2 cases IgM was observed on the patients'' platelet membranes. C3 [complement component 3] was not detected on patients'' platelets. Platelet-associated IgG was also found in several other disorders and its occurrence is not therefore of ITP. Serum platelet antibodies do not indicate specifically ITP, as they may also be due to previous isoimmunization. Antibodies in sera of patients with ITP generally did not fix C1q in most cases bound to platelets only in the presence of EDTA. Isoantibodies often fixed C1q and had equal affinity for platelets suspended in ACD [acid-citrate-dextrose] or EDTA plasma. This was confirmed by quantitative data on IgG binding by platelets obtained by measuring 125I-labeled protein A uptake. The simplicity of the IF technique permits its routine application and the technique may give useful information with respect to the nature of the antibodies. It must be considered of limited value in the diagnosis of ITP.