Quantitation of antigen-specific IgG in human serum: Standardization by a Staph A solid phase radioimmunoassay elution technique

A new solid phase method for quantitation of antigen-specific IgG in human serum is described. Antigen insolubilized on agarose is used in sufficient quantity to remove >90% of specific IgG from an aliquot of human serum. From 30 to 60% of antigen-specific IgG is then eluted from the washed antibody-antigen-sorbent complex by means of alkaline pH treatment (pH 10 to 11, 30 min, 25°C). The total IgG content of the elute is determined by a sensitive solid phase RIA by using ¹²⁵I-Staphylococcal protein A (Staph A), which binds avidly to >93% of human IgG. The partially eluted antigen-sorbent together with a sham eluted control is then washed and incubated further with Staph A in order to calculate the elution efficiency. With these data, it is then possible to calculate IgG antibody content in weight/volume terms. This method was evaluated in the quantitation of IgG antibody specific for porcine insulin in the sera of two insulin-resistant diabetics and for honeybee phospholipase A (PLA) in serum from five PLA-allergic patients. Results from this method were compared with those obtained independently by Scatchard analysis of the binding of radiolabeled antigen in a radioimmunoprecipitation technique. Agreement between the two methods was excellent (mean coefficient of variation between techniques = 8.9%). Furthermore, values determined directly by the Staph A solid phase elution technique (SPET) agreed well with those obtained by interpolation from the standard curve of a reference serum of known specific IgG content. This suggests that primary standardization of a single reference serum by the Staph A SPET may be sufficient.