Immune thrombocytopenic purpura. Use of a 125I-labeled antihuman IgG monoclonal antibody to quantify platelet-bound IgG.

We used a radiolabeled monoclonal antibody to the Fc portion of human IgG to measure platelet-bound IgG in 54 thrombocytopenic patients and in 48 normal controls. The control group had a mean (+/- 1 S.D.) of 169 +/- 79 IgG molecules bound per platelet--10 to 100 times lower than values generally reported with other assay techniques. Sixteen of 17 patients with untreated thrombocytopenic purpura considered to be of immune origin had values ranging from 790 to 13,095--levels well above those found in normal subjects and in patients with "nonimmune" thrombocytopenia (246 +/- 156). In a second group, consisting of 11 patients with "immune" thrombocytopenic purpura who did not respond to conventional therapy (corticosteroids or splenectomy), the values were quite divergent: two patients had levels in the normal range, three had levels in the range of subjects with nonimmune thrombocytopenia, and six had much higher levels. Measurement of platelet-bound IgG with this monoclonal-antibody technique can help differentiate between immune and nonimmune thrombocytopenias. Low platelet counts in patients who have immune thrombocytopenia according to clinical criteria but are resistant to therapy may be related to factors other than IgG antibody.