IHC and FISH Testing: How Labs Measure HER2

HER2 is a protein that can influence how breast cancer grows, and labs measure it with two main tests: IHC (immunohistochemistry), which looks at how much HER2 protein sits on the surface of cancer cells, and FISH (fluorescence in situ hybridization), which counts copies of the HER2 gene inside the cells. IHC is usually done first; FISH is often used when the IHC result is unclear or needs confirmation.

Both tests are run on tissue already collected during a biopsy or surgery, so no new procedure is needed. Understanding what each test measures can make the HER2 line on your pathology report much easier to read.

Key Takeaways

HER2 is a growth-related protein; some breast cancers make extra amounts of it, and labs test for this routinely.
IHC measures the HER2 protein on the cell surface and reports a score from low to high; it is the usual first test.
FISH measures the HER2 gene itself by counting gene copies inside the cells; it is often used to clarify an in-between IHC result.
Both tests use tissue already collected — you do not need a new biopsy for HER2 testing.
Results are reported as positive, negative, or needing clarification, and they feed into the molecular subtype of the cancer.
Because testing details vary, your own care team is the right source for what your result means for your situation.

What HER2 Is and Why It Is Tested

Every cell in the body carries instructions in its genes, and one of those genes is called HER2. It produces a protein that sits on the cell surface and helps tell the cell when to grow and divide. In some breast cancers, the cells carry extra copies of the HER2 gene and make much more of the protein than usual. When that happens, the cancer is described as HER2-positive.

Knowing the HER2 status is a standard part of a breast cancer diagnosis because it is one of the three main markers — along with the estrogen and progesterone receptors — that define the cancer’s molecular subtype. The result helps the care team understand the cancer’s biology and shapes which treatment conversations are relevant. HER2 testing is done on the same tissue removed during a biopsy or surgery, and it is routine enough that you may not hear about it until the results appear in your report.

It is worth knowing that HER2 status is about the cancer cells, not about you. A HER2-positive result does not mean you carry a hereditary gene change, and it does not change your family members’ risk. It is simply a description of what the cancer cells are doing.

What IHC Measures

IHC — immunohistochemistry — is the test labs usually run first. A thin slice of the tissue sample is treated with special antibodies that attach to HER2 protein on the cell surface, along with a stain that makes the attached antibodies visible. A pathologist then looks at the slide and judges how much staining is present.

The result is reported on a scale that runs from low to high:

Low scores mean little or no HER2 protein was detected on the cells. These results are generally reported as HER2-negative.
High scores (often written as 3+) mean the protein is strongly present across many cells. These results are generally reported as HER2-positive.
In-between scores (often written as 2+) mean the staining is moderate — not clearly low, not clearly high. These results are considered borderline and usually need a second, different test to settle the question.

IHC is relatively quick and inexpensive as lab tests go, which is why it comes first. But because it relies on a pathologist’s visual judgment of staining, borderline cases get passed along to FISH for a more precise measurement.

What FISH Measures

FISH — fluorescence in situ hybridization — takes a different approach. Instead of looking at protein on the cell surface, it looks at the gene itself. Fluorescent probes attach to the HER2 gene inside the cell nucleus, and the lab counts how many copies of the gene each cell carries. Extra copies of the gene — a situation called gene amplification — generally line up with the extra protein that IHC detects.

The FISH result is reported as positive (amplified) or negative (not amplified), based on how the counted copies compare with the lab’s standard reference. Because it involves counting rather than judging stain intensity, FISH is often described as the more precise of the two tests.

FISH takes longer than IHC and costs more, which is why it is not always the first test. It is most often ordered when the IHC result lands in the borderline zone, though some labs run it alongside IHC from the start, or repeat it when a result needs confirmation. Either way, it uses the same tissue sample — there is nothing extra for you to do.

A nurse offering a comforting hand on a patient's shoulder in a bright waiting area — A nurse offering a comforting hand on a patient’s shoulder in a bright waiting area

IHC vs FISH: A Side-by-Side Look

Both tests answer the same question — is this cancer HER2-positive? — but they measure different things to get there:

Feature	IHC	FISH
What it measures	HER2 protein on the cell surface	Copies of the HER2 gene inside the cell nucleus
How it works	Antibodies and stain highlight the protein; a pathologist judges the staining	Fluorescent probes mark the gene; the lab counts gene copies
Result format	A score from low to high (low, in-between, or high)	Positive (amplified) or negative (not amplified)
When it is used	Usually the first HER2 test	Often after a borderline IHC, or alongside IHC for confirmation
Speed and cost	Faster and less expensive	Slower and more expensive, but more precise
Sample needed	The same biopsy or surgery tissue — no new procedure	The same tissue — no new procedure

Think of IHC as the screening step and FISH as the clarifying step. Most of the time, a clearly low or clearly high IHC score settles the matter. FISH steps in for the cases in the middle — and occasionally to double-check a result the lab wants to be certain about.

When Results Are Unclear

Sometimes the two tests do not line up neatly, or a result comes back borderline even after FISH. This happens for ordinary reasons: the tumor may contain a mix of cells with different HER2 levels, the sample may be small, or the tissue processing may have affected the staining. Labs deal with these situations regularly, and an unclear result is not a sign that something went wrong.

When results are uncertain, the usual next steps are straightforward:

Re-testing the same sample. The lab may repeat the test on the same tissue block to rule out a technical issue.
Testing a different sample. If surgery follows the biopsy, the surgical tissue may be tested, giving the lab more material to work with.
A second look. Another pathologist may review the slides for an independent read.

Waiting for a clarified result can be frustrating, especially when the rest of the plan feels ready to move. It helps to remember that the care team would rather wait for an accurate HER2 result than build a plan on a guess. If the delay is weighing on you, ask how long the re-test is expected to take and what can be prepared in the meantime. Our complete guide to how breast cancer is found and diagnosed explains where HER2 testing fits in the overall diagnostic sequence.

Reading the HER2 Line on Your Report

The HER2 result usually appears in the biomarker or receptor section of the pathology report, near the estrogen and progesterone receptor results. Here is how to decode what you see:

“HER2 negative” or a low IHC score: the lab found little HER2 protein — the cancer is considered HER2-negative.
“HER2 positive” or a high IHC score (often 3+): the lab found strong HER2 protein presence — the cancer is considered HER2-positive.
An in-between IHC score (often 2+) with a FISH result: the final call comes from the FISH result — “amplified” means HER2-positive, “not amplified” means HER2-negative.
“HER2-low”: a newer category describing cancers with a small amount of HER2 protein — not enough to be HER2-positive, but not zero either. If you see this term, ask your care team what it means for your situation.
“Pending” or “addendum to follow”: the IHC is done but the FISH (or another clarifying test) is still running.

Reports sometimes use slightly different wording from lab to lab, so if the phrasing on yours does not match these descriptions exactly, bring the report to your appointment and ask. For the broader context of what HER2 status changes, see our guides on HER2-positive vs HER2-negative breast cancer and triple-positive breast cancer.

A husband and wife sitting close together on a couch at home in the evening

Questions to Ask Your Doctor

HER2 results deserve a clear explanation in plain language. These questions can help:

What is my HER2 status, and which test (IHC, FISH, or both) was used to determine it?
Was my result clearly positive or negative, or was it borderline — and if borderline, how was it clarified?
How does my HER2 result fit with my hormone receptor results to define my cancer’s subtype?
Does my HER2 status affect which treatment options we will discuss?
Will the HER2 testing be repeated on any tissue removed during surgery?
If I want to read more, which organizations’ information do you recommend?

Individual cases differ, and no general guide can interpret your report for you. Your care team’s read of your results is the one that counts.

Frequently Asked Questions

Will I need another biopsy for HER2 testing?

No. IHC and FISH are both performed on tissue already collected during your biopsy or surgery. The lab works from the same tissue block, so HER2 testing requires no additional procedure on your part.

How long does HER2 testing take?

IHC results often come back within several days, while FISH can take longer — sometimes a week or more — because the process is more involved. If FISH is ordered after a borderline IHC, the total wait is the two waits added together. Your care team can usually give you a rough timeframe.

What does “equivocal” mean on an HER2 report?

“Equivocal” is lab language for borderline or uncertain — the result fell in the middle range and could not be called clearly positive or negative. An equivocal IHC result is the most common reason FISH gets ordered. The FISH result then provides the final answer.

Can HER2 status change?

HER2 results can occasionally differ between the original biopsy sample and tissue removed later, which is why testing is sometimes repeated. If the cancer comes back after treatment, re-testing may also be discussed. Ask your care team whether your HER2 result was confirmed on more than one sample.

Is HER2-positive breast cancer hereditary?

HER2 status describes the cancer cells, not your inherited genes. Being HER2-positive does not mean you carry a hereditary mutation, and it does not by itself change your family members’ risk. Questions about hereditary risk are separate and are usually addressed through family history and, when appropriate, genetic counseling.

This article is for general educational purposes only and is not medical, legal, or financial advice. It does not replace professional care. Talk to your doctor or care team about your own situation. If you think you may have a medical emergency, call 911.