An enzyme‐linked immunosorbent assay for the quantification of serum platelet‐bindable IgG

Abstract

An enzyme-linked immunosorbent assay (ELISA) using F(ab'')2 peroxidase-labeled antihuman Ig and o-phenylenediamine dihydrochloride (OPD) as a substrate was developed to measure serum platelet bindable IgG (S-PBIgG). The assay was made quantitative by standardizing the number of normal target platelets bound to microtiter plate wells, and by incorporating quantitated IgG standards with each microtiter plate tested to prepare a standard calibration curve. By this method, S-PBIgG for normal individuals was 3.4 .+-. 1.6 fg/platelet (mean .+-. 1 SD; n = 40). Increased S-PBIgG levels were detected in 36 of 40 patients with clinical autoimmune thrombocytopenia (ATP), ranging from 7.0-85 fg/platelet. Normal S-PBIgG levels were found in 34 of 40 patients with nonimmune thrombocytopenia. This method showed a sensitivity of 90%, specificity of 85% and in the sample population studied, a positive predictive value of 0.86 and a negative predictive value of 0.90. This assay is highly reproducible (coefficient of variation was 6.8%) and appears useful in the evaluation of patients with suspected immune-mediated thrombocytopenia.