Estradiol receptor assays in breast cancer.

The ER (estradiol receptor) assay using radioactively labelled estradiol is used to determine if a patient with breast cancer will respond to endocrine therapy. A positive ER value will elicit a response rate between 50%-60% as compared to a 30% response rate among patients without regard to the ER value. To determine ER value cytosol is obtained from centrifuged tissue radioactive estradiol is added and the concentration of the receptor is determined by the amount of radioactive estradiol that binds to it. When the percent positive rate is significantly less than what it is expected faulty tissue handling should be considered a possibility.