An ultrasensitive radioligand assay for IgG using the protein A on Staphylococcus aureus bacteria

The purpose of this study was to develop a simple and sensitive assay to measure IgG. Human IgG was radiolabelled with 125Iodine and 7.5 ng was incubated with heat-killed Staphylococcus aureus bacteria (Cowan 1 strain). To replicate sets of tubes, increasing amounts of a standard IgG preparation were added. The samples were incubated at room temperature for two hours and separated by centrifugation. Using this assay it was found that the IgG concentration could readily be determined in one nanoliter or less of human serum. There was no significant cross-reactivity with IgA, IgE and IgM or the F(ab')2 fragment of IgG. Serial dilutions of normal human or SLE sera, rabbit or guinea pig sera, the Fc fragment of human IgG and a mouse monoclonal anti-human DNA antibody parallelled the dose response curve obtained with standard human IgG. The method correlated well (r=0.89) with a routinely used nephelometric method. The mean (+/- SD) IgG concentration in 20 normal subjects measured by this assay was 10 +/- 3.6 g/L.