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If the IHC result is definitive, overall interpretation is based on that IHC result. In other words, if HER2 IHC is 0 or 1+, the overall interpretation is negative. If HER2 IHC is 3+, the overall interpretation is positive.¹ If HER2 IHC is 2+, as it so often is in cases sent for ISH, additional ISH cells should be counted by a second observer, blinded to the original ISH results, but with attention to the 2+ or strongest staining areas of the carcinoma (Table 2 for this example). If the result remains the same, the overall interpretation is HER2 negative, as delineated (Table 3).¹

The overall interpretation for a 2+ IHC result differs by ISH group, as summarized in Table 3 and fully detailed in the guideline.

*If ISH result is different based on additional cell counts, results are adjudicated based on standard procedures and used for overall interpretation. †See guideline for additional details.¹

1. Wolff AC, Hammond MEH, Allison KH, et al. Human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline focused update. Arch Pathol Lab Med. 2018;142(11):1364–1382.

Megan Troxell, MD, PhD
Professor, Department of Pathology
Stanford University School of Medicine, Stanford, Calif.
Advisor, CAP Immunohistochemistry Committee

Andrew Michael Bellizzi, MD
Clinical Associate Professor, Department of Pathology, University of Iowa Hospitals and Clinics
Co-director of Immunopathology Laboratory, Director of GI Pathology, Holden Comprehensive Cancer Center, Iowa City, Iowa
Chair, CAP Immunohistochemistry Committee