Development of Standard Estrogen and Progesterone Receptor Immunohistochemical Assays for Selection of Patients for Antihormonal Therapy

Estrogen receptor (ER) and progesterone receptor (PR) status in breast carcinomas are considered validated predictive factors for selecting patients for antihormonal therapy. Published surveys have shown a significant rate of disagreement and lack of reproducibility of immunohistochemistry (IHC) results from laboratories around the world. To address these limitations IHC assays for ER and PR were developed using characterized reagents, after careful calibration of the sensitivity and specificity to match established assays previously validated in large clinical studies. The ER assay uses a cocktail of 2 mouse monoclonal antibodies (1D5 and ER-2-123) and the PR assay uses 1 mouse monoclonal antibody (PgR 1294); both are followed by a polymer-peroxidase-based detection system. All antibodies were tested for specificity by epitope mapping. The sensitivity of the new assays was calibrated to be equivalent to previously validated IHC assays followed by a comparison with the validated assays in a concordance study involving over 200 specimens. All slides were scored with the "Allred Score," also used for scoring of the original validated assays. The overall concordance between the new and the established IHC assays was nearly perfect (99%). The concordance study demonstrated greater than 98% positive agreement and 100% negative agreement of the new IHC assays with the previously validated IHC assays. This equivalence establishes the clinical validation of the assays and, as they are based on newer generation reagents and are produced and tested under stringent quality control conditions to ensure their consistency, they add additional advantages to the user and patients.