HER2 Equivocal: What It Means for Your Diagnosis

An HER2 equivocal result means the first-line test on your tumor tissue could not clearly determine whether the cancer overexpresses a growth-promoting protein called HER2. Scored as 2+ on an immunohistochemistry (IHC) staining test, equivocal falls in a gray zone between clearly negative (0 or 1+) and clearly positive (3+). The result does not mean your diagnosis is uncertain or that something has gone wrong. It means an additional test, usually a type of genetic probe called FISH or another in situ hybridization method, is needed to settle the question, and for an increasing number of patients, that in-between zone itself carries treatment relevance.

What the IHC Scores Actually Tell You

When a pathologist examines a breast cancer biopsy for HER2 status, the standard first step is immunohistochemistry. The test stains tumor cells to reveal how much HER2 protein sits on their surfaces. Results are reported on a four-tier scale. A score of 0 means no staining is detected. A score of 1+ means faint or barely visible staining. A score of 3+ means strong, complete staining of the cell membranes across a large fraction of tumor cells, which counts as HER2-positive. And then there is 2+, the equivocal score, defined as weak to moderate complete membrane staining in more than 10% of the tumor cells.¹

The trouble with the 2+ category is that it captures a wide range of biology. Some 2+ tumors turn out to have genuine HER2 gene amplification once a second test is run, making them functionally HER2-positive. Others have no amplification at all and behave more like HER2-negative cancers. A small fraction land in yet another gray zone on the genetic test. Because IHC alone cannot sort these cases, guidelines from the American Society of Clinical Oncology and the College of American Pathologists direct labs to automatically send all 2+ results for reflex testing with in situ hybridization.²

The Reflex Test and What Its Results Mean

Fluorescence in situ hybridization, or FISH, works differently from IHC. Instead of looking at the protein on the cell surface, it counts the actual number of HER2 gene copies inside the cell’s nucleus. It also counts copies of a reference marker on chromosome 17 so that the ratio between the two can be calculated. If the HER2-to-chromosome-17 ratio is 2.0 or higher with an average of four or more HER2 copies per cell, the tumor is classified as ISH-positive, meaning HER2-amplified. If the ratio is below 2.0 and the copy number is below four, it is ISH-negative. Between those poles sits another equivocal zone: a ratio below 2.0 but an average copy number between four and six.³

Most equivocal IHC 2+ cases resolve to HER2-negative on FISH, meaning the gene is not amplified. A smaller proportion turn out to be truly amplified and therefore eligible for traditional HER2-targeted therapy. When the FISH result itself is equivocal, guidelines recommend additional testing steps or a repeat on a new sample. Tumors with borderline copy numbers closer to 6 are more likely to eventually test positive than those near the lower end of the range.⁴

The reflex testing process adds days or sometimes weeks to the turnaround. For patients waiting on results, that delay can feel agonizing. But the wait is usually straightforward, and your oncologist will not finalize a treatment plan until the full HER2 picture is clear.

Why HER2 Status Matters So Much for Treatment

HER2 is a receptor protein that, when overexpressed, sends signals pushing tumor cells to grow and divide aggressively. Cancers with HER2 overexpression tend to be more proliferative and, historically, were linked to worse outcomes before targeted therapies came along.⁵ The development of drugs that specifically block or exploit this receptor, starting with trastuzumab (Herceptin) in the late 1990s, transformed HER2-positive breast cancer from one of the most dangerous subtypes into one of the most treatable. In a pivotal trial, patients with HER2-amplified tumors who received trastuzumab plus chemotherapy survived roughly 50% longer than those on chemotherapy alone.⁶

Getting HER2 status right has enormous consequences in both directions. Missing a truly HER2-positive cancer means a patient does not receive a therapy that could substantially extend their life. Incorrectly labeling a cancer as HER2-positive exposes a patient to expensive treatment with side effects but no meaningful benefit. One economic analysis estimated that among women diagnosed with early-stage breast cancer in a single year in the United States, roughly 7,000 received false-positive HER2 results and about 5,000 received false-negatives, collectively costing society nearly a billion dollars in lost quality of life and misallocated treatment.⁷

The Rise of HER2-Low and What It Means for Equivocal Results

For decades, HER2 testing was essentially binary: positive or negative. If your tumor was IHC 2+ but FISH-negative, it was grouped with 0 and 1+ as simply “HER2-negative,” and that was the end of the story. Newer drugs have changed that. A class of treatments called antibody-drug conjugates, most prominently trastuzumab deruxtecan (brand name Enhertu), can deliver a chemotherapy payload directly to cells expressing even small amounts of HER2. The DESTINY-Breast04 trial showed that in metastatic breast cancers classified as HER2-low (IHC 1+ or IHC 2+ with negative FISH), trastuzumab deruxtecan nearly doubled progression-free survival compared to standard chemotherapy, extending overall survival to about 24 months versus roughly 17.5 months.⁸

This has given the equivocal IHC 2+ category a new clinical weight. A 2+ result that resolves to FISH-negative no longer means “same as zero.” It means HER2-low, a designation that opens the door to antibody-drug conjugate therapy in the metastatic setting. Meanwhile, a 2+ result that resolves to FISH-positive means HER2-positive, qualifying for the traditional anti-HER2 drugs as well as newer ones. Either way, the equivocal score is now more actionable than it has ever been.

The 2023 ASCO/CAP guideline update acknowledged this shift, noting that the distinction between IHC 0 and IHC 1+ has become clinically relevant for the first time because of trial entry criteria for trastuzumab deruxtecan, even though the guidelines stopped short of formally creating new result categories like “HER2-low.”⁹ Researchers are also exploring an even finer distinction, “HER2-ultralow,” for tumors that show the faintest hint of staining below what currently qualifies as 1+. These categories are still evolving, but the direction is clear: the old binary is giving way to a spectrum.¹⁰

Why Equivocal Results Happen in the First Place

Some tumors genuinely sit at the biological boundary. They express moderate amounts of HER2 protein, enough to stain under a microscope but not enough for a definitive call. This biological reality is compounded by technical factors that can push a result toward or away from the 2+ threshold.

One major variable is how the tissue sample is handled before it ever reaches the pathologist’s slide. The time between surgical removal and placement in fixative (called cold ischemia time), the duration of fixation, and even the concentration or type of formalin used can all alter how HER2 protein shows up on IHC staining. Studies in both breast and gastric cancer models have shown that delays in fixation or the use of non-standard formalin concentrations can shift IHC scores, sometimes enough to flip a result from positive to equivocal or from equivocal to negative.¹¹ ASCO/CAP guidelines specify that tissue should be fixed in 10% neutral buffered formalin for 6 to 72 hours, and deviations from this can affect accuracy.¹² Research comparing standard fixation with alternative fixatives has found that when conditions deviate from the recommended protocol, a portion of HER2-positive cases shift to HER2-negative readings.¹³

Pathologist interpretation also introduces variability. Scoring IHC is partly subjective: deciding whether membrane staining is “weak,” “moderate,” or “strong” involves human judgment. Training programs try to standardize this, and pathologists who score HER2 receive specific instruction on the fine distinctions between 0, 1+, and 2+.¹⁴ But inter-observer variability persists, particularly at the boundaries between adjacent scores. This is another reason reflex testing exists: the second test uses objective gene counting rather than visual protein-staining assessment.

Tumor Heterogeneity and Shifting HER2 Status

A fact that surprises many patients is that HER2 status can differ within different parts of the same tumor, a phenomenon called intratumoral heterogeneity. Some clusters of cells may strongly express HER2 while neighboring clusters do not. This patchwork pattern can make the IHC score depend partly on which area of tissue the pathologist happens to examine, and it has been linked to poorer responses to anti-HER2 therapies.¹⁵

HER2 status can also change over time or between the primary tumor and a metastasis. A study using 386 paired samples of primary tumors and recurrences found that about a quarter of cases showed a shift in HER2 classification, with most conversions being between the “zero” and “low” categories.¹⁶ When researchers specifically compared primary breast tumors to liver metastases, the heterogeneity rate was even higher, with roughly 38% of cases showing a mismatch.¹⁷ Separate research across multiple receptor types found HER2-specific discordance in a smaller fraction of cases when using the traditional positive-versus-negative cutoff, suggesting the rate of discordance rises when finer distinctions like HER2-low are tracked.¹⁸

These findings are a practical argument for retesting HER2 when cancer recurs or spreads. Your original biopsy may tell one story while the new site tells another, and your treatment options depend on the current chapter.

Prognostic Signals in the Equivocal Category

An IHC 2+ result that turns out FISH-negative is generally grouped with HER2-negative cancers for treatment purposes, but emerging evidence suggests it may not behave identically to a 0 or 1+ tumor. A study of older patients with high-risk, HER2-negative breast cancer found that those whose tumors were scored 2+ with negative FISH had significantly worse disease-free survival compared to those scored 0 or 1+, with the equivocal-but-unamplified group carrying a hazard ratio above five for recurrence in multivariate analysis.¹⁹ This is a single study in a specific population, so it would be premature to generalize broadly, but it reinforces the idea that the 2+/FISH-negative group is not biologically identical to tumors with no HER2 expression at all.

HER2 Testing Beyond Breast Cancer

Breast cancer gets the most attention in discussions of HER2 equivocal results, but HER2 overexpression also plays a role in gastric and gastroesophageal cancers, where targeted therapy with trastuzumab has become part of standard care for advanced disease.²⁰ The scoring system differs slightly: gastric cancers are allowed to count incomplete membrane staining as positive at 3+ because of the glandular architecture of the stomach lining, which means the IHC interpretation rules are not interchangeable with breast cancer. Separate guidelines from the College of American Pathologists and ASCO address testing algorithms, scoring, and clinical decision-making specifically for gastroesophageal adenocarcinoma.²¹ If you received an equivocal HER2 result on a gastric or esophageal biopsy, the same basic principle applies: reflex testing with in situ hybridization is the next step, but your oncologist will use the gastric-specific scoring criteria.

Artificial Intelligence and More Precise Scoring

Because so much rides on distinguishing IHC 0 from 1+ from 2+, and because human scoring of faint staining is inherently subjective, there is growing interest in using AI to assist pathologists. An algorithm tested for its ability to distinguish HER2 IHC 0 from 1+ boosted pathologist accuracy from about 80% to 93% and improved scoring consistency across different observers.²² That distinction matters more now than it used to, precisely because the boundary between 0 and 1+ determines eligibility for HER2-low therapies. AI tools are not replacing pathologists, but they are being studied as a second pair of eyes that could reduce the rate of borderline calls and lower the number of patients whose treatment hinges on a subjective visual impression.

What Patients Often Do Not Know

A survey-based study found that among breast cancer patients who knew their HER2 status, only about half were at least somewhat familiar with the HER2-low category. Among patients with known HER2-negative disease, roughly half did not recall HER2-low ever being discussed with them, even though most had been tested within the past year. When patients who did not know their HER2 status at diagnosis were given brief educational material, about 61% expressed an intention to bring up HER2-low testing with their oncologist.²³

If your pathology report says IHC 2+, it is worth asking your oncologist a few specific questions. First, has reflex FISH testing been ordered, and what were the results? Second, if FISH is negative, does your cancer qualify as HER2-low, and does that change your treatment options now or potentially in the future? Third, if your cancer recurs, will HER2 be retested on the new tissue? These are not questions most patients think to ask, but in the current treatment landscape, the answers can open doors to therapies that did not exist a few years ago.

When Selective or Repeat Testing Comes Into Play

Some clinical scenarios complicate the standard IHC-then-FISH pathway. When metastatic breast cancer is biopsied and scores IHC 2+ but the primary tumor was clearly HER2-negative, researchers have suggested that selective rather than automatic reflex ISH testing may be appropriate if the IHC is carefully reviewed, since the resource-intensive FISH step rarely flips the result in that context.²⁴ This is an area of active debate rather than settled practice, and for most patients, full reflex testing remains standard. UK guidelines have also outlined situations where repeat HER2 testing from a fresh sample is advisable, particularly when genetic heterogeneity is suspected or when results from a small biopsy are inconsistent with the clinical picture.

The 2023 ASCO/CAP update continued to recommend that the distinction between IHC 2+ with FISH-negative and lower IHC scores be tracked carefully in pathology reports, even though the formal treatment categories have not yet been officially redrawn.²⁵ For you as a patient, this means asking for the specific IHC score and the FISH ratio, not just a “positive” or “negative” summary. That detail may matter for clinical trial eligibility or future treatment decisions.