Best practices for achieving consensus in HER2‐low expression in breast cancer: current perspectives from practising pathologists

Aims Human epidermal growth factor receptor 2 (HER2) expression is an important biomarker in breast cancer (BC). Most BC cases categorised as HER2‐negative (HER2−) express low levels of HER2 immunohistochemistry (IHC) 1+ or IHC 2+/ in‐situ hybridisation not amplified (ISH−) and represent a clinically relevant therapeutic category that is amenable to targeted therapy using a recently approved HER2‐directed antibody–drug conjugate. A group of practising pathologists, with expertise in breast pathology and BC biomarker testing, outline best practices and guidance for achieving consensus in HER2 IHC scoring for BC. Methods and results The authors describe current knowledge and challenges of IHC testing and scoring of HER2‐low expressing BC and provide best practices and guidance for accurate identification of BCs expressing low levels of HER2. These expert pathologists propose an algorithm for assessing HER2 expression with validated IHC assays and incorporate the 2023 American Society of Clinical Oncology and College of American Pathologist guideline update. The authors also provide guidance on when to seek consensus for HER2 IHC scoring, how to incorporate HER2‐low into IHC reporting and present examples of HER2 IHC staining, including challenging cases. Conclusions Awareness of BC cases that are negative for HER protein overexpression/gene amplification and the related clinical relevance for targeted therapy highlight the importance of accurate HER2 IHC scoring for optimal treatment selection.