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EBioMedicine2025;120105919.October 1, 2025Journal Article

New standards in HER2-low testing: the CASI-01 comparative methods study.

David J Dabbs1, Emina Torlakovic2, Søren Nielsen3, Suzanne C Parry4, Jing Yu5, Catherine Stoos6, Beth Clark5, Henrik Høeg7, Jeppe Thagaard7, Seshi R Sompuram8, Stephen P Naber9, Yukako Yagi10, James Sayre11, Kodela Vani8, Mélissande Cossutta12, Francoise Soussaline12, Alexandre Papine12, Nils A t'Hart13, Matthias J Szabolcs14, Bharat Jasani15, Mary Kinloch2, Luis Chiriboga16, Keith Miller17, Steve Bogen18
1CASI Steering Committee Member, USA; Department of Pathology, University of Pittsburgh Medical Center, Pittsburgh, PA, USA.
2CASI Steering Committee Member, USA; Department of Pathology and Laboratory Medicine, University of Saskatchewan & Saskatoon Health Authority, Royal University Hospital, Saskatoon, Saskatchewan, Canada.
3CASI Steering Committee Member, USA; NordiQC, Department of Pathology, Aalborg University Hospital, Aalborg, Denmark.
4CASI Steering Committee Member, USA; Department of Immunocytochemistry and In Situ Hybridisation, UKNEQAS-ICC/ ISH, London, United Kingdom.
5Department of Pathology, University of Pittsburgh Medical Center, Pittsburgh, PA, USA.
6Department of Pathology, Nebraska Methodist Health System, Omaha, NE, USA.
7Visiopharm A/S, Hørsholm, Denmark.
8Boston Cell Standards Inc., Boston, MA, USA.
9Department of Pathology & Laboratory Medicine, Tufts Medical Center, Boston, MA, USA.
10Department of Pathology, Memorial Sloan Kettering Cancer Center, New York, NY, USA.
11Department of Biostatistics, University of California (UCLA), Los Angeles, CA, USA.
12Imstar Dx, Paris, France.
13CASI Steering Committee Member, USA; Department of Pathology, Isala, Zwolle, the Netherlands.
14CASI Steering Committee Member, USA; Department of Pathology & Laboratory Medicine, New York-Presbyterian/Columbia University Irving Medical Center, New York, NY, USA.
15CASI Steering Committee Member, USA; Division of Cancer and Genetics, School of Medicine, Cardiff University, Heath Park, Cardiff, CF14 4XN, United Kingdom.
16CASI Steering Committee Member, USA; Department of Pathology & Center for Biospecimen Research and Development, NYU Grossman School of Medicine, New York, NY, USA.
17CASI Steering Committee Member, USA; Department of Research Pathology, The Cancer Institute, University College London, United Kingdom.
18CASI Steering Committee Member, USA; Boston Cell Standards Inc., Boston, MA, USA; Department of Pathology & Laboratory Medicine, Tufts Medical Center, Boston, MA, USA. Electronic address: sbogen@bostoncellstandards.com.
PubMed PMC DOI PDF

Abstract

BACKGROUND: The introduction of Trastuzumab deruxtecan (T-Dxd) has exposed clinically significant limitations in accurately detecting HER2-low expression testing when using immunohistochemistry (IHC) assays originally developed to detect HER2 over-expression. While HER2 testing is widely used to determine T-Dxd eligibility, no HER2-low assay was ever validated against HER2 protein expression. MET…

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