The development and application of a serum assay for platelet-bindable IgG (S-PBIgG)

Direct assays for PAIgG are useful in diagnosing ITP and investigating mechanisms of thrombocytopenia. A serum assay for S-PBIgG would offer technical advantages in transportation and storage of samples and could prove useful in the diagnosis of thrombocytopenia caused by alloantibodies. We report the development of an assay for S-PBIgG that is positive in the majority of patients with immune thrombocytopenia. This was accomplished by empirically testing a number of variables, including the anticoagulant used to prepare target platelet, the presence of a fixative, incubation time, and the target platelet count. The development of this assay was facilitated by using a microtiter modification of the antiglobulin consumption assay, which was validated by simultaneously determining PAIgG with each method (r = 0.91, p less than 0.01, n = 42). The serum assay was then applied to the study of several types of immune thrombocytopenia PAIgG and S-PBIgG were simultaneously quantitated on platelets and sera obtained from 42 patients with idiopathic thrombocytopenia. PAIgG was elevated in 38 and S-PBIgG elevated in 34. Eleven patients had a normal or only moderately elevated S-PBIgG but a considerably elevated PAIgG. In these patients the unbound platelet antibody or immune complex may have platelet specificity. In the remaining patients there was a close correlation between the level of PAIgG and S-PBIgG (r = 0.81, n = 31) indicating that the bound platelet antibody or immune complex is dynamic equilibrium with the unbound. The potential application of the assay for S-PBIgG was demonstrated in three leukemic patients who were refractory to platelet transfusions and in two mothers who had infants with alloimmune neonatal thrombocytopenia. In these patients elevated S-PBIgG was associated with a normal or only slightly elevated PAIgG.