HER2 Testing in Breast Cancer (ASCO-CAP)
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Direct answer
Three scores decide whether a breast cancer patient receives HER2-directed therapy, and each has a written definition. Immunohistochemistry comes first: 3+ (complete, intense membrane staining in over 10% of tumour cells) is positive; 1+ (faint, barely perceptible incomplete staining in over 10%) and 0 are negative; 2+ is equivocal and goes to reflex dual-probe in-situ hybridisation. The 2023 ASCO/CAP update reaffirmed the 2018 scoring rules but made one distinction clinically load-bearing: IHC 0 versus 1+, because IHC 1+ and 2+/ISH-negative ("HER2-low") tumours qualify for trastuzumab deruxtecan in the metastatic setting.
What you must remember
- IHC 3+: complete, intense membrane staining in over 10% of invasive tumour cells — report HER2 positive, no ISH needed.
- IHC 2+ (equivocal): weak complete membrane staining in over 10% of cells, or complete intense staining in 10% or fewer; reflex to ISH, and reporting states the percentage for both patterns when present.
- ISH groups (dual probe): ratio 2.0 or more — amplified (group 1); ratio under 2.0 with average HER2 copy number under 4.0 — negative (group 4); ratio under 2.0 with copies 4.0 to under 6.0 — negative (group 2, the 2018 change); ratio under 2.0 with copies 6.0 or more — adjudicate with IHC: 3+ makes it positive, 0/1+ makes it negative (group 3).
- HER2-low: IHC 1+ or IHC 2+ with negative ISH — not a new breast cancer class per the guideline, but an eligibility state for trastuzumab deruxtecan after chemotherapy in metastatic HR-positive disease.
- HER2-ultralow: IHC 0 with faint incomplete staining in 1-10% of cells — an observational concept in the update, relevant to trial eligibility, without mandated separate reporting.
- Preanalytics: 10% neutral buffered formalin, cold ischaemia under 1 hour, fixation 6-72 hours; crushed, cauterised or edge tissue is avoided.
- 2023 additions: testing of the metastatic or recurrent specimen is recommended (with consent), acknowledging inter-tumoural heterogeneity and HER2 gain on recurrence.
An equivocal case walked through
A core biopsy shows invasive ductal carcinoma with weak complete membrane staining in about 30% of cells: IHC 2+, so the laboratory runs dual-probe ISH and counts 20 non-overlapping nuclei. Average HER2 signals come back 7.2 with average CEP17 at 4.0 — ratio 1.8, copy number 7.2. Ratio under 2.0 but copies 6.0 or more: group 3, the ambiguous group. The guideline resolves it by returning to the IHC — if the case were 3+, report positive; here it is 2+, so the laboratory counts a further 20 cells from the area with strongest HER2 signal and reports the group honestly. If the second count holds group 3 with IHC 2+, the result stays equivocal-negative pending multidisciplinary discussion — the exact scenario where clinical trial data and the treating oncologist's judgement, not a reflex label, decide therapy. Every element of this walk — the 20-cell count, the group assignment, the IHC tie-break — is quotable in an exam answer.
Where students slip
Basolateral or incomplete intense staining is not 3+, and cytoplasmic staining is ignored entirely — two scoring errors that over-treat patients. The second slip is calling HER2-low a new subtype; the guideline explicitly declined to make it a separate class of breast cancer, framing it instead as an eligibility descriptor for antibody-drug conjugates. Third, a ratio of 2.0 or more is amplified even when the absolute copy number is low, which contradicts intuition and is a beloved MCQ. Fixation questions close the loop: under-fixation or over-fixation produces false negatives, and the 6-72 hour window is as much a HER2 rule as a scoring rule.
Frequently asked questions
What makes an IHC score 3+?
Circumferential, intense, complete membrane staining in more than 10% of invasive tumour cells — positive without further testing.
What does the laboratory do with IHC 2+?
Reflex dual-probe ISH with a 20-cell count; the HER2/CEP17 ratio and average copy number assign one of four ISH groups.
Is HER2-low a new subtype of breast cancer?
No — the guideline affirms it is not a separate class, but IHC 1+ or 2+/ISH-negative status does establish eligibility for trastuzumab deruxtecan in metastatic disease.
How is ISH group 3 resolved?
Ratio under 2.0 with average HER2 copy number of 6.0 or more is adjudicated by IHC: 3+ is positive, 0 or 1+ is negative.
Why retest on the metastatic biopsy?
Recurrences can shift HER2 status through heterogeneity or acquisition of amplification, so the 2023 update recommends testing the recurrent or metastatic sample with informed consent.