What Does an Equivocal or Conflicting HER2 Test Mean?

An equivocal or conflicting HER2 test means that your breast cancer’s HER2 status isn’t definitively positive or negative on the initial test, or that results differ between different tissue samples ^[1][2]. This does not mean the lab made a mistake; rather, it reflects the complex biology of the tumor or variations in tissue handling ^[3]. To clarify the result, your care team will use follow-up testing (such as reflex testing) or review a different tissue block ^[4][5]. Understanding this status is an important step in your care, as it is a key factor—alongside your cancer stage, hormone receptor (ER/PR) status, and overall health—in determining your treatment options ^[6][7].

Making Sense of the Scores (IHC and ISH)

When evaluating invasive breast cancer, the pathologist first performs an Immunohistochemistry (IHC) test to measure the amount of HER2 protein on the surface of the cancer cells ^[4].

• IHC 0 or 1+: Considered HER2-negative ^[4][5].
• IHC 2+: The equivocal (borderline) zone ^[4]. It shows a moderate amount of HER2 protein, but not enough to be clearly positive.
• IHC 3+: Strongly positive ^[7].

If your score is IHC 2+, medical guidelines recommend a second test called In-Situ Hybridization (ISH) (sometimes called FISH) ^[5][4]. While IHC looks at proteins, ISH looks inside the cell’s nucleus to see if there are extra copies (amplification) of the HER2 gene ^[4]. Reflex ISH is the usual confirmatory step, and it may be performed on the same tissue block or another suitable specimen. By integrating both the IHC and ISH results, the pathologist can classify the tumor as definitively HER2-positive or HER2-negative ^[4].

(Note: Final interpretation assumes standard test conditions and pathologist review ^[4][5][7])

Why Might Tests Conflict or Disagree?

Sometimes, a core biopsy might test negative, but the surgical removal shows a positive or borderline result, or vice versa ^[2]. Conflicting results across tests or over time can happen for several medical and technical reasons:

• Intratumoral Heterogeneity: A tumor is not always uniform. One area might have high HER2 levels while another area has low levels ^[1][2].
• Changes Over Time: If cancer spreads to another part of the body (metastatic), or if you undergo treatments like chemotherapy, the tumor’s biology can evolve ^[8]. Retesting a metastatic lesion may be considered when feasible to see if the HER2 status has changed.
• Tissue Handling: How the tissue is preserved (fixation) and processed in the lab can influence the test’s chemical reactions ^[3].

If a core biopsy comes back negative but the cancer has unusual features or the results don’t match the clinical picture, doctors may test the larger surgical specimen ^[2]. A surgical result doesn’t automatically override a biopsy; pathologists look at the quality and findings of both samples to determine the most accurate diagnosis ^[2].

Understanding “HER2-Low” and “HER2-Ultralow”

Historically, breast cancer was strictly divided into HER2-positive and HER2-negative. Recently, treatment guidelines have recognized sub-categories within the traditional “HER2-negative” group that may respond to newer drugs:

• HER2-Positive: IHC 3+, or IHC 2+ with a positive (amplified) ISH test ^[7]. Treated with standard HER2-targeted therapies.
• HER2-Low: IHC 1+, or IHC 2+ with a negative (non-amplified) ISH test ^[9].
• HER2-Ultralow: IHC 0, but with faint membrane staining ^[10][11].

It is crucial to understand that HER2-low and HER2-ultralow tumors are not treated as HER2-positive with standard anti-HER2 regimens ^[9]. However, for selected patients with unresectable or metastatic disease, newer medications called antibody-drug conjugates (ADCs)—which act as targeted carriers that deliver chemotherapy directly to cancer cells—may be an option ^[6][11].

Drugs like trastuzumab deruxtecan have shown the ability to improve outcomes for certain patients with advanced HER2-low or ultralow disease ^[6][11]. Eligibility for these therapies depends on multiple factors, including whether the disease is metastatic, your hormone-receptor (ER/PR) status, prior treatments, and overall health. These drugs are not an automatic recommendation and carry risks, including serious lung inflammation (interstitial lung disease/pneumonitis) ^[6].

What Happens Next?

Waiting for clarification on borderline or conflicting results can be highly stressful. However, there is a standard medical workup for these situations, which may involve repeating the IHC or ISH on a different tissue block, reviewing a metastatic sample, or seeking an expert pathology review ^[4][2]. Your care team will ensure the right combination of tests is used and interpreted within the broader context of your overall health and cancer stage.