A comparative study on estradiol receptor assays in human breast cancer tissue

Standard dextran-coated charcoal assays for measuring the estrogen receptor content in human breast cancer tissue are compared. A single-point assay, using the estrogen antagonist nafoxidine as competitive inhibitor, showed good qualitative agreement with a three-point Scatchard plot assay, using radio-inert estrogen to assess the non-specific binding. Based on statistically evaluated dividing points, 116 (66.3%) of the 175 analysed primary tumor specimens were positive in the single-point assay compared to 121 (69.1%) in the Scatchard plot assay. Use of the additional dissociation constants in the evaluation of the Scatchard plot assay data gives a complete qualitative agreement between the two assays. We conclude that, in spite of the lack of specific controls inherent in the Scatchard plot assay, the single-point assay using diethylstilbestrol instead of nafoxidine is equally useful, especially if biopsy specimens are too small to provide the larger number of aliquots necessary for multiple steroid receptor analyses. Factors affecting the quantitative differences between the two assays are discussed.