Platelet-associated complement C3 in immune thrombocytopenic purpura.

Platelet-associated C3 (PA-C3) was measured with a quantitative immunofluorescence assay. With this assay, PA-C3 levels were determined for 78 normal volunteers, 30 patients with immune thrombocytopenic purpura (ITP), and 20 patients with nonimmune thrombocytopenias. Platelet-associated IgG (PA-IgG) levels were also measured with our standard quantitative immunofluorescence assay. All patients with nonimmune thrombocytopenias and ITP in remission had normal PA-C3 levels. Twenty-four patients with active ITP were classified into 3 groups: 9 (38%) with increased PA-IgG and normal PA-C3 levels, 10 (42%) with elevated PA-C3 and PA-IgG levels, and 5 (20%) with increased PA-C3 values only. A direct correlation was found between PA-C3 and PA-IgG levels. PA-IgG levels were higher in the group of patients with elevated PA-C3 levels than in those with normal values. Platelet survival studied showed reduced survival times of 1.5--2.5 days for the 5 patients with elevated PA-C3 levels only. Elevated PA-C3 levels returned to normal in 7 ITP patients whose platelet counts increased in response to corticosteroid therapy or to splenectomy. Therefore, PA-C3 and PA-IgG assays can be used to identify patients with ITP, to follow their response to therapy, and to classify them into immunologic subgroups similar to red cell classification by Coombs' testing in immune hemolytic anemia.