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Quantitation of Estrogen Receptor in Seventy-five Specimens of Breast Cancer: Comparison between an Immunoassay (Abbott ER-EIA Monoclonal) and a [³H]Estradiol Binding Assay Based on Isoelectric Focusing in Polyacrylamide Gel¹

Department of Clinical Chemistry [Å. P., S. A. G.], Department of Pathology [P. S.], and Department of Medical Nutrition [A. M. T., J-Å. G.], Karolinska Institute, Huddinge University Hospital, S-141 86 Huddinge, Sweden, and Department of Pathology [A. N., J. S., A. L.], Akademiska sjukhuset, S-751 85 Uppsala, Sweden

Quantitation of estrogen receptor has been performed in cytosol prepared from 75 specimens of breast cancer tissue from patients who had not received hormonal therapy. The study was performed in order to compare an immunoassay (Abbott Laboratories, North Chicago, IL) with our currently used method for estrogen receptor analysis based on isoelectric focusing of [³H]estradiol-receptor complex in polyacrylamide gels. Using linear regression analysis, a regression coefficient (slope) of 1.30 and a correlation coefficient of 0.75 were calculated. The differences in results between the two methods are probably partly explained by the fact that the ligand-based method only measures unoccupied receptor, whereas the immunoassay detects the total amount of receptor, resulting in generally slightly higher concentrations with the latter method. However, in five of 75 specimens the ligand-based method gave a considerably higher concentration of estrogen receptor. This was most probably explained by partial proteolysis resulting in the formation of receptor fragment(s), which was undetectable with the immunoassay but detectable with the ligand-based method. These observations underline the importance of careful handling of specimens during the whole immunoassay procedure.