Key Insights

• Understand how this test reveals your tumor’s biology—whether HER2 is driving cancer cell growth through extra receptors or gene amplification.
• Identify receptor status (HER2-positive, HER2-negative, or HER2-low) that can explain fast-growing tumors, clarify prognosis, and shape treatment pathways.
• Learn how tumor genetics, sampling site, prior therapy, and lab method (immunohistochemistry vs in situ hybridization) influence results and their meaning.
• Use insights to guide targeted therapy discussions, surgical and chemotherapy planning, and clinical trial eligibility in partnership with your oncology team.
• Track how results change over time to monitor disease evolution or re-test a recurrence or metastasis when biology may have shifted.
• When appropriate, integrate this test with hormone receptor status (ER/PR), proliferation markers, or genomic profiling to build a complete picture of breast cancer behavior.

What Is a HER2 Test?

A HER2 test measures whether breast cancer cells make too much of the HER2 protein (a cell-surface growth receptor) or carry extra copies of the ERBB2 gene that codes for it. Most often, the sample is tumor tissue from a core needle biopsy or surgery. Pathologists assess HER2 protein by immunohistochemistry (IHC) and, when needed, confirm gene status with in situ hybridization (ISH), such as fluorescence in situ hybridization (FISH). Results are reported as categories (IHC 0, 1+, 2+, or 3+; ISH amplified or not amplified) against established guidelines to determine HER2-positive, HER2-negative, or HER2-low status. Some centers also use next-generation sequencing or, in select metastatic settings, blood-based assays, but tissue HER2 testing remains the standard for clinical decision-making.

Why this matters: HER2 status reflects how aggressively a tumor is signaling for growth and how likely it is to respond to HER2-targeted therapies. It connects to core systems like cell proliferation, survival signaling, and metastatic potential. Testing provides objective data at diagnosis and at progression to uncover drivers of disease, refine prognosis, and map a precise treatment strategy—often before symptoms or imaging tell the full story.

Why Is It Important to Test Your HER2?

HER2 is like a growth “antenna” on a breast cancer cell. When there are too many antennas (protein overexpression) or too many instruction manuals for making them (ERBB2 gene amplification), the cell hears louder signals to divide, invade, and resist normal checks and balances. A her2 test reveals whether this pathway is in the driver’s seat. That single insight ties to practical realities: tumors with high HER2 activity can grow and spread faster, but they are also uniquely targetable with HER2-directed therapies. In other words, knowing HER2 turns a blurry picture into a map you can act on with your care team.

Testing is especially relevant at the time of diagnosis; whenever biopsy shows invasive breast cancer; when cancer recurs or metastasizes (biology can change); and when pathology is borderline on routine staining. It is also important in less common scenarios—such as male breast cancer—because HER2 can still be the key growth signal. Pathology reports translate this biology into clear categories. IHC 3+ is typically HER2-positive; IHC 2+ prompts confirmatory ISH testing; amplified ISH results also indicate HER2-positive disease. IHC 0–1+ is considered HER2-negative, and a subset within this range is now described as HER2-low (IHC 1+ or 2+ with negative ISH), a classification that has emerging treatment implications though ongoing research continues to refine best practices.

Zooming out, HER2 testing is central to prevention-minded oncology: it helps catch high-risk biology early, guides whether HER2-targeted agents are likely to help, and provides a baseline for tracking how a tumor adapts under treatment. Repeating the test on a new site of disease can reveal a shift in receptor status—something we see in real-world cohorts—which can open or close specific therapeutic options. The goal is not to “pass” or “fail,” but to surface the right information at the right time so your plan fits the tumor you have today. Clinical guidelines from expert groups have standardized interpretation to reduce guesswork, though nuanced cases still benefit from subspecialty pathology review.

What Insights Will I Get From a HER2 Test?

Your report shows how strongly the tumor expresses the HER2 protein (IHC score 0, 1+, 2+, or 3+) and, when indicated, whether the ERBB2 gene is amplified by ISH (often reported as a ratio of HER2 signals to a reference such as CEP17). “Normal” here means levels typical of non–HER2-driven tumors, while “positive” indicates biologically meaningful overexpression or amplification that often responds to HER2-targeted therapy. Context matters: an IHC 2+ result is indeterminate on its own and gains meaning only with the ISH result.

Balanced or negative values suggest the tumor is not primarily relying on the HER2 pathway. That can point your team toward other drivers—like hormone receptors or specific genomic alterations—when planning therapy. Variation is expected across patients and even within the same tumor; factors like sample handling, prior therapy, and tumor heterogeneity can influence results.

Higher values (IHC 3+ and/or ISH-amplified) indicate HER2-driven biology that may be more aggressive but also more targetable. Lower values (IHC 0–1+) indicate HER2-negative disease; HER2-low (IHC 1+ or 2+ with negative ISH) reflects minimal protein expression without gene amplification. Abnormal does not equal “worst” or “best” on its own—these categories guide further evaluation and, with your clinician, inform a tailored plan.

The real power is in pattern recognition over time. Interpreted alongside estrogen and progesterone receptor status, tumor grade, stage, imaging, and—when appropriate—genomic profiling, HER2 results help reveal trends such as stable biology versus evolution under treatment. That perspective supports early detection of change, smarter sequencing of therapies, and personalized decisions that align with your goals and life.