A microtiter solid-phase radioimmunoassay for platelet-associated immunoglobulin G.

An MSPRIA is described for measuring platelet-associated immunoglobulins by competitive inhibition of the binding of radiolabeled goat anti-human class-specific antibody to solid-phase immunoglobulins. PAIgG can be estimated from calibration curves constructed from soluble IgG inhibitors. The MSPRIA for IgG is antibody class-specific. The coefficient of variability is between 6% and 20%. The MSPRIA is more sensitive than other described assays. Comparative studies demonstrated that ACD-A is the preferred anticoagulant for collecting platelets for the MSPRIA and that sonicated platelets are more reliable than intact platelets in demonstrating elevated PAIgG in immune thrombocytopenias. Intact platelets from 18 normal volunteers had PAIgG ranging from 0.4 to 3.3 fg/platelet (mean +/- S.D. = 1.5 +/- 1). The same platelets when sonicated had a mean +/- S.D. PAIgG of 3.3 +/- 1 fg/platelet, with a range of 1 to 5. Only 60% of 30 adult ITP patients with presumed immune thrombocytopenia had elevated PAIgG levels when their intact platelets were studied. When these same platelets were sonicated, 87% of them had abnormal levels of PAIgG.