Validation of EP1 Antibody Clone for Estrogen Receptor Immunohistochemistry for Breast Cancer

Background: SP1 Rabbit monoclonal antibody to estrogen receptor (ER) has long been the standard for determination of ER status in breast cancer but has been replaced by the rabbit EP1 clone. Aim: To validate the EP1 antibody clone for use in determination of breast cancer ER status in a large clinical population against the previous standard SP1. Materials and Methods: ER immunohistochemistry was assessed in 523 consecutive cases from a clinical setting using tissue microarrays. Results: The kappa statistic showed that the agreement of ER status between SP1 and EP1 was considered to be almost perfect (kappa=0.97, 95% confidence interval=0.94-1.00). Sensitivity was 99.3%, specificity was 98.6% and overall agreement was 99.2%. Conclusion: The EP1 antibody was herein validated regarding its use in breast cancer with almost perfect agreement with the previously used standard SP1 antibody.