Research ArticlesWhat to Expect From the New ASCO/CAP Guideline Recommendations for Hormone Receptor Testing in Breast Cancer: A National Reference Laboratory ExperienceMahlow, Jonathon MD; Goold, Eric A. MD; Jedrzkiewicz, Jolanta MD; Gulbahce, H. Evin MD, MSCIAuthor Information Department of Pathology, University of Utah, Salt Lake City, UT Presented in part at the United States and Canadian Academy of Pathology Annual Meeting, March 3, 2020, Los Angeles, CA. The authors declare no conflict of interest. Reprints: H. Evin Gulbahce, MD, MSCI, Department of Pathology, Huntsman Cancer Institute, University of Utah, 2000 Circle of Hope, Room 3100, Salt Lake City, UT 84112 (e-mail: [email protected]). Applied Immunohistochemistry & Molecular Morphology: April 2021 - Volume 29 - Issue 4 - p 245-250 doi: 10.1097/PAI.0000000000000891 Buy Metrics Abstract The new ASCO/CAP guidelines on hormone receptor testing in breast cancer recommends standard operating procedures (SOPs) established to confirm or adjudicate estrogen receptor (ER) results with weak or ≤10% staining, and the status of internal controls (ICs) reported for cases with 0% to 10% staining. The aim of this study is to determine the frequency of ER testing with weak or ≤10% staining that may require additional steps following SOPs and to identify any correlation between hormone receptor status of the tumor and the likelihood of finding IC. Breast cancer cases between January 2014 and April 2019 were included to identify negative, low-positive and weak-positive cases. The presence/absence of IC was correlated to tumor type. Following ASCO/CAP guidelines, 29.8% of cases (374/1261) will need additional steps to confirm/adjudicate results due to negative, low, or weak positive ER status. The probability of finding IC is ~50% lower in cases of ER and progesterone receptor (PgR) negative tumors. Repeat testing may be warranted in 13.1% (92/700) of all cases due to lack of IC. In conclusion, the new ASCO/CAP guidelines recommend laboratories to establish and follow SOP to confirm or adjudicate ER results for about 30% of the cases before reporting hormone receptors status. Over 40% of cases with <10% tumor ER positivity lacked IC that may need a comment per the guidelines indicating a repeat testing may be warranted. However, the presence/absence of IC may be related to the subtype of breast cancer and should not necessarily bring into question the validity of the test. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.