Human epidermal growth factor receptor 2 (HER-2) is the prognostic and predictive biomarker for breast cancer found in a quarter of all breast cancer cases. Precise testing of HER-2 will impact treatment response.Based on the updated American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) HER-2 testing guidelines, we report 141 cases of positive concordance rate between HER-2 IHC stain and dual-probe ISH for HER-2 testing classified in equivocal (2+) and positive (3+) IHC groups in Nakhon Pathom Hospital, Thailand. Our study showed statistical significance in the relationship between positive (3+) IHC and dual-probe ISH for HER-2 testing with a correlation rate of 91.76% (rs = 0.38031, p < 0.0001) and 37.50% in equivocal (2+) IHC groups.This is the first report of a positive concordance rate between equivocal and positive HER-2 IHC and dual-probe ISH for HER-2 based on the ASCO/CAP 2018 guidelines in Thailand. Our study confirmed a statistically significant relationship in the positive IHC (3+) group. In addition, we suggested reflexing ISH testing only in equivocal IHC (2+) cases to decrease the waiting time and economical approach.
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