HER2 testing: evolution and update for a companion diagnostic assay

Human epidermal growth factor receptor 2 (HER2; encoded by ERBB2) testing has been a cornerstone of patient selection for HER2-targeted therapies, principally in breast cancer but also in several other solid tumours. Since the introduction of HercepTest as the original companion diagnostic for trastuzumab, HER2 assessment methods have evolved substantially, incorporating various testing modalities, from western blots, immunohistochemistry and fluorescence in situ hybridization, to early chromogenic quantitative methods and, probably in the future, fully quantitative methods. The advent of highly effective HER2-targeted antibody-drug conjugates with clinical activity at low levels of HER2 expression, such as trastuzumab deruxtecan, has necessitated the re-evaluation of HER2 testing, particularly for HER2-low tumours. In this Review, we provide an in-depth overview of the evolution of HER2 testing, the current clinical guidelines for HER2 testing across various solid tumours, challenges associated with current testing methodologies and the emerging potential of quantitative techniques. We discuss the importance of accurately defining HER2-low expression for therapeutic decision-making and how newer diagnostic approaches, such as quantitative immunofluorescence and RNA-based assays, might address the limitations of traditional immunohistochemistry-based methods. As the use of HER2-targeted therapies continues to expand to a wider range of tumour types, ensuring the precision and accuracy of HER2 testing will be crucial for guiding treatment strategies and improving patient outcomes.