Quantitative determination of antibody in idiopathic thrombocytopenic purpura. Correlation of serum and platelet-bound antibody with clinical response.

We studied the clinical applicability of a recently developed technic that determines antiplatelet antibody directly on the platelet surface or in serum. The technic is a quantitative complement lysis-inhibition assay. Normal platelets have less than 0.4 pg of surface IgG. All patients with idiopathic thrombocytopenic purpura who were studied had greater than that value. Surface IgG was increased in inverse proportion to the platelet count. Surface levels of greater than 1.1 pg correlated with failure to respond to prednisone therapy. Incubation of normal serums with normal platelets did not increase surface IgG of such platelets, but the incubation with thrombocytopenic serums increased their surface IgG 0.5 to 100 times. The degree of increase did not predict response to treatment. However, quantitation of surface IgG of thrombocytopenic platelets was useful in predicting response to treatment.