HER2-low breast cancer: what the test result means for treatment
HER2-low describes a breast cancer test result with some HER2 protein expression that does not meet the usual HER2-positive threshold. It can matter when selecting an antibody-drug conjugate for advanced disease. It does not by itself determine the entire treatment plan. Hormone receptor status, disease stage, previous endocrine therapy or chemotherapy and the exact HER2 test result remain important. Ask the team to connect the report with a specific treatment decision.
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Read the score behind the label
HER2-low generally means immunohistochemistry, or IHC, 1+, or IHC 2+ with a negative in situ hybridization test. HER2-positive disease meets different criteria. The laboratory report should include the actual score and the confirmatory result when needed.
Ask for the original report rather than relying on a summary that says HER2-negative. That older category can include cancers with low expression. A pathologist can clarify the wording if the report does not give enough detail for a current treatment assessment.
What HER2-ultralow adds
HER2-ultralow is a more specific category of IHC 0 with membrane staining used in a defined advanced breast cancer treatment setting. It differs from IHC 0 with no membrane staining. Distinguishing these low levels depends on the testing and interpretation.
If an oncologist is considering a treatment linked to ultralow expression, ask whether the existing sample and report are adequate. The practical question is whether a pathology review or another specimen would change a treatment option, not simply whether a new label can be added.
Sources and references: [1]
Hormone receptor status still guides the plan
A cancer can be HER2-low and hormone receptor-positive. In that situation, endocrine-based treatment can remain important. HER2-low can also occur in hormone receptor-negative disease, but the evidence and prior-treatment requirements for an ADC may differ.
Ask the oncologist to explain how ER and PR results combine with the HER2 score. A low HER2 result should not lead to abandoning an effective endocrine approach without a reason related to progression, eligibility or the overall treatment setting.
When trastuzumab deruxtecan may be relevant
Trastuzumab deruxtecan is an ADC with evidence in selected advanced HER2-low breast cancer settings. In January 2025, the US FDA expanded its indication to unresectable or metastatic HR-positive, HER2-low or HER2-ultralow disease that has progressed on one or more endocrine therapies in the metastatic setting.
This provides a concrete example of why a detailed HER2 report matters. The treating service must apply the relevant local indication and the patient's prior treatment history. Ask which eligibility criteria you meet and what alternative is being compared with the ADC.
A low score does not automatically change early treatment
The evidence and indications discussed above concern advanced disease. An early breast cancer plan still depends on stage, hormone receptors, recurrence risk and the treatments supported in that setting. A HER2-low result alone is not a general reason to use an advanced-disease ADC regimen after surgery.
Ask whether the result changes anything you need to do now. If a clinical trial is proposed, request the study details and the standard option outside the trial. This separates a current treatment decision from a possible future option.
Why the specimen and timing can matter
Receptor expression can differ between samples or change over time. If cancer has recurred, a metastatic sample may help reassess the current biology when obtaining tissue is safe and useful. The team also needs to consider the quality and interpretation of the test.
Bring the reports from both the original tumor and any later biopsy. Ask which specimen is being used to support the recommendation and whether an expert review of stored tissue would answer the question without another procedure.
Sources and references: [2]
Discuss lung symptoms and other treatment risks
Trastuzumab deruxtecan can cause lung inflammation, including serious interstitial lung disease or pneumonitis. A new cough, breathlessness or fever needs prompt contact with the treatment team. Nausea and changes in blood counts can also require support and monitoring.
Tell the oncologist about prior lung disease and previous treatment-related breathing problems. Ask for the safety monitoring schedule and what happens if a symptom develops between infusions. The decision should compare expected disease control with the risks most relevant to your health.
Sources and references: [2]
Preparing a HER2-low consultation in Shanghai
Fudan University Shanghai Cancer Center has a breast cancer multidisciplinary service. A focused review can clarify whether the existing pathology supports an ADC discussion and how it fits the treatment sequence.
EastMedGo can help organize the inquiry with the original HER2 reports, ER/PR results, scans and prior medicines. Tell us which treatment has been proposed or which result is unclear. We can clarify appointment logistics, tissue-transfer requirements when requested by the service and coordination costs.
Sources and references: [4]
Common questions
Is HER2-low the same as HER2-positive?
No. The testing thresholds and treatment implications differ. Keep the IHC score and any ISH result in your records.
Does HER2-low always mean an ADC should start now?
No. Stage, hormone receptors, prior therapy, health and the relevant indication determine whether and when an ADC is appropriate.
Sources and references: [1]
Sources and references
- FDA approves fam-trastuzumab deruxtecan-nxki for unresectable or metastatic HR-positive, HER2-low or HER2-ultralow breast cancer
- Breast Cancer Treatment (PDQ) – Health Professional Version
- Hormone Therapy for Breast Cancer
- Fudan University Shanghai Cancer Center: Breast Cancer multidisciplinary team
Prepared with AI assistance by EastMedGo for patients comparing treatment and care options.