A double antibody radioassay for the detection of platelet associated IgG

A double antibody radioimmunoassay has been developed for the detection of platelet associated IgG (PAIgG). The assay employs conventional radioassay technology and, as such, readily may be adopted by laboratories familiar with radioassay techniques. The assay is sensitive to 0.6 ng IgG and can be undertaken on 10(6)-10(7) platelets, or fewer in the case of thrombocytopenia with raised PAIgG. The assay can be completed within 2 working days. Normal PAIgG levels in EDTA blood were 0.8 to 5.7 micrograms IgG/10(9) platelets. However, normal levels depended upon the anticoagulant used for blood collection and were on average five-fold higher for blood taken into EDTA than for blood taken into ACD; normal range 0.1-1.2 micrograms/IgG 10(9) platelets. PAIgG was assayed on blood taken simultaneously into ACD and EDTA from 15 patients with chronic idiopathic thrombocytopenia (ITP). PAIgG was raised in 14 out of 15 EDTA samples, but in only 11 of 15 ACD samples. EDTA was therefore regarded as the anticoagulant of choice. A total of 20 of 23 (87%) patients with chronic ITP had PAIgG values above the normal range. The PAIgG in chronic ITP was weakly related to platelet count, weakly and inversely related to platelet volume, but was independent of serum IgG concentration.