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HER2 is scored 0 to 3+ on the stain; 3+ is positive, 0 and 1+ are negative, and 2+ goes on for a gene test. The UK reads one borderline gene result as positive where the American guideline reads it as negative, so the same tumour can be HER2-positive in Britain and HER2-negative in the United States.
HER2 status has to be known for every invasive breast cancer, and the dataset puts the frequency of HER2 positivity in early breast cancer at 13 to 20 percent. The UK recommendation is a two-tier system: immunohistochemistry first, with reflex in situ hybridisation for the equivocal cases, or a one-tier in situ hybridisation strategy where the quality of fixation is doubtful. Only membrane staining of the invasive tumour is scored; cytoplasmic staining, staining of in situ disease and normal epithelium are not, and the staining that counts for 3+ must be intense and uniform, described in the dataset as resembling chicken wire. A score of 3+ is unequivocally positive, 0 and 1+ are negative, and 2+ is equivocal and mandates in situ hybridisation. Where technical problems stop either test being reported, the answer is indeterminate and the test is repeated, not guessed.
The UK recommendation is to use dual-probe in situ hybridisation and to report three numbers: the ratio of HER2 signals to chromosome 17 signals, the HER2 copy number and the chromosome 17 copy number, counted in 20 to 60 nuclei across at least three tumour fields. A ratio of 2.0 or more, or a mean HER2 copy number of 6 or more, is positive, and a copy number of 6 or more with a ratio below 2 is also positive. Then comes the divergence. Cases with a ratio of 2.0 or more but a HER2 copy number below 4, the group ASCO and CAP call FISH group 2, are classified as negative by the American guideline. The UK guidance keeps them positive, on the evidence of a retrospective series in which IHC 2+ tumours with a ratio of 2.0 or more and copy number below 4 responded to neoadjuvant therapy much as other tumours with a ratio of 2.0 or more did. So a report from a British laboratory and a report from an American one can disagree about the same tumour, and a patient reading American material about her own result should know it.
The rest of the guidance is about not being fooled. A borderline negative ratio of 1.8 to 1.99 on a needle biopsy should be repeated on the surgical specimen. HER2 should be reassessed on a surgical specimen if the biopsy was uninterpretable, if the invasive tumour on the core was too small, if the resected tumour looks morphologically different from the core, or if the core showed strong HER2 staining in under 10 percent of the invasive cancer, which may be genuine intratumoral heterogeneity. Pre-analytics matter more here than in almost any other test: the time between removing the tissue and putting it in formalin should be under an hour, cores need at least six hours of fixation and surgical specimens 24 to 72 hours, sections should be stained within a day or two of cutting because excessive drying loses HER2 expression, and decalcified bone biopsies may need the gene test because the acid damages the protein stain. As with the hormone receptors, the HER2 result from a commercial multigene assay must not be used to manage a patient.
Shares How ER and PR are scored on a breast report (Allred score, H score, and why PR is not a UK core item), HER2-low eligibility for trastuzumab deruxtecan, and TROP2 ADCs without a test (triple-negative disease), Receptor conversion: when the receptors change between the primary and a recurrence, HER2-low and HER2-ultralow.
Shares How ER and PR are scored on a breast report (Allred score, H score, and why PR is not a UK core item), Receptor conversion: when the receptors change between the primary and a recurrence, Grade, stage and receptor status: three different things on one breast report, HER2-positive (IHC 3+ or ISH-amplified).
Shares HER2-low and HER2-ultralow metastatic breast cancer, HER2-low and HER2-ultralow, Immunohistochemistry (IHC), Breast cancer (all types).
Shares HER2-positive (IHC 3+ or ISH-amplified), HER2-low and HER2-ultralow metastatic breast cancer, HER2-low and HER2-ultralow, FISH / ISH (in situ hybridisation).
Shares HER2-low and HER2-ultralow metastatic breast cancer, HER2-low and HER2-ultralow, FISH / ISH (in situ hybridisation), Immunohistochemistry (IHC).
Shares Grade, stage and receptor status: three different things on one breast report, The WHO classification of breast tumours, and what the 6th edition changed, HER2-positive breast cancer, Breast cancer (all types).
Shares Grade, stage and receptor status: three different things on one breast report, The WHO classification of breast tumours, and what the 6th edition changed, HER2-positive breast cancer, Breast cancer (all types).
Shares HER2-low and HER2-ultralow metastatic breast cancer, HER2-low and HER2-ultralow, Immunohistochemistry (IHC), Breast cancer (all types).