Objectives: Our study aims to review the common practice of retesting HER2 in breast Excision Specimens (ES) that tested negative in Core Biopsies (CB) for its potential significance and effectiveness. Methods: Our Quality Assurance (QA) data was queried for all breast cancers with HER2 tests from 2015-2021. Patients with negative HER2 results (defined as IHC score 0, 1+, and 2+/FISH-) in Core Biopsies (CB) and subsequent HER2 testing in Excisional Specimens (ES) of the same breast cancer were included in the study. Results: A total of 548 cases with HER2 negative results (IHC 0 or 1+, n = 522; IHC 2+/FISH-, n = 26) on CB were found, and the results of their repeat HER2 tests in subsequent ES were reviewed. 32 out of 522 (6%) cases were upgraded, which included 31 cases from the negative category (IHC 0, n = 7, and IHC 1+, n = 24) on CB specimens to the equivocal IHC 2+ category on ES, and a single case of IHC 1+ to the IHC 3+ category. FISH was performed on all 58 cases (31 upgraded IHC2+, 1 upgraded IHC 3+, and 26 IHC 2+). With the 2018 updated HER2 FISH criteria, only 3/548 (0.54%) cases were FISH amplified. The final conversion rate from negative HER2 to a positive status between CB and ES is only 0.54% in our cohort, with non-descriptive HER2 retesting in ES. Conclusions: The overall discordance of HER2 status between CB and ES is only 0.54%. Hence, the CB test results should be considered reliable for pre-operative patient management. Although rare, the converted HER2 result in retesting could significantly impact the post-operative management of individual patients. Our study provides data for analyzing the practicality of this common practice of retesting HER2 in ES for tumors with negative HER2 test in CB, thereby discouraging retesting in low-grade disease as per ASCO/ CAP recommendation.