PURPOSE: The recent approval of antibody drug conjugates (ADCs) targeting HER2 (like Trastuzumab Deruxtecan or T-DXd) have led to challenges for the immunohistochemical (IHC) companion diagnostic test since the test was optimized for gene amplified levels of HER2. Here we develop and validate an objective test for low level HER2 expression toward more accurate selection of patients for T-DXd. EXPERIMENTAL DESIGN: We validated the high-sensitivity HER2 assay using a mix of the requirements for an IHC assay and that of a ligand binding assay. Then we prospectively tested it on 316 core biopsy specimens received by Yale Pathology Labs from August 2022 to August 2023. RESULTS: Using a 40-case breast cancer tissue validation set, we find very high accuracy and precision with a coefficient of variation below 10% and define a reportable range for the assay in amol/mm CONCLUSION: This assay validation study provides a method for accurate, objective measurement of HER2 and has the potential to improve selection of patients for T-DXd or similarly targeted ADC therapies in the future. TRANSLATIONAL RELEVANCE: Antibody Drug Conjugates (ADCs) represent a novel class of targeted therapy, but in some ADC approvals the target is not assessed due to the lack of an objective and reproducible method for measurement of protein on slides. This paper describes, validates and tests a new fully quantitative method for ADC target assessment on histology slides.