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FISH and IHC Testing for HER2: Test Length, Results, and 5 Other Facts

Medically reviewed by Maybell Nieves, M.D.
Written by Maureen McNulty
Updated on May 12, 2025

​Most people living with breast cancer have tests to determine whether their cancer is HER2-positive or HER2-negative. Knowing your HER2 status early is important because it helps you and your doctor decide which treatments are most likely to work.

Between 15 percent and 20 percent of people with breast cancer have HER2-positive tumors, according to the American Society of Clinical Oncology (ASCO). This means their tumor cells have higher-than-normal levels of the HER2 gene (also called human epidermal growth factor receptor 2, HER2/neu, or ERBB2).

Read more about the genetics of HER2-positive breast cancer.

1. It Begins With Breast Cancer Diagnosis

Receiving a breast cancer diagnosis often begins with discovering a lump or mass in the breast. This might happen during a clinical exam, a breast self-exam, or a routine mammogram screening.

Imaging tests — such as ultrasound or MRI — can help get a closer look at a lump, but they can’t confirm whether it’s cancer. A biopsy is usually needed to make a diagnosis.

2. Biopsy Is the First Step in HER2 Testing

A biopsy is often the first step in both breast cancer diagnosis and HER2 testing. During a biopsy, a doctor will take a small sample of tissue from your breast for microscopic study.

A biopsy is often the first step in both breast cancer diagnosis and HER2 testing.

The biopsy can show whether the sample contains breast cancer cells (also called breast carcinoma) and help identify the cancer type and how advanced it is. It can also reveal genetic mutations (changes) in the cancer cells, including whether they have high levels of the HER2 gene. This information helps your doctor create the most effective treatment plan.

3. There Are Different Types of Biopsies

Doctors can use different methods to collect a sample of breast tissue. The type of biopsy recommended depends on factors such as the size and location of the lump, as well as your preferences and medical history.

Needle-Based Biopsy

If your doctor suspects breast cancer, they will probably recommend a core needle biopsy. During this procedure, the doctor numbs part of the breast, makes a small cut in the skin, and removes several small, cylinder-shaped pieces of tissue using a hollow needle.

Another possible biopsy option is a fine-needle aspiration (FNA). This uses a thinner needle and removes fewer cells. FNA is usually quick and may not require numbing medication — it typically feels similar to getting an injection.

During needle-based biopsies, the doctor needs to make sure they are putting the needle into the right area and removing the right tissue. They may do this by feeling for the lump, or they may use additional imaging tests to guide the needle — the most common is ultrasound guidance.

After a needle biopsy, the doctor or nurse may apply a dressing. The area usually heals quickly, though some swelling or minor bleeding may occur. Follow your care team’s instructions for how to care for the biopsy site.

Surgical Biopsy

Surgical breast biopsies are less common than core needle biopsies. They may be used if a needle biopsy doesn’t provide enough information or can’t access the lump.

During a surgical breast biopsy, a doctor removes all or part of the lump. This procedure typically takes place in a hospital. You’ll likely receive medication to help you relax or sleep and to prevent pain.

4. Depending on Biopsy Results, You May Need More Testing

In most cases, a biopsy shows that the tissue is not cancerous. But if breast cancer is found, the biopsy can reveal important details, including what type of cancer it is, how aggressive it may be, and how far it might have spread.

Once your biopsy sample is collected, it’s sent to a laboratory, where it’s examined by a doctor called a pathologist (a specialist trained to study cells under a microscope). It usually takes a few days to get the results.

Your doctor can tell you when to expect your results and how you’ll receive them. The pathologist will look for several markers in the tissue, including whether the cancer cells produce certain proteins like HER2.

5. Cancer Cells Are Graded Based on How They Look Under a Microscope

The pathologist will likely place some of the cells on a glass slide and use a microscope to view them more closely and determine whether they are cancerous.

Healthy cells tend to be uniform and neatly arranged. Cancer cells, in contrast, are more likely to be disorganized and form unusual-looking structures. They might also show signs of spreading into nearby tissues. These features help the pathologist tell whether the sample contains normal or cancerous cells.

The pathologist assigns a breast cancer grade based on how different the cancer cells look from normal breast cells.

  • Low grade cancer cells look more like normal cells and tend to grow more slowly.
  • High-grade cancer cells look more abnormal and are expected to grow and spread more quickly.

Your doctor can use information about your cancer’s grade along with your HER2 status as predictive factors to estimate your prognosis (outlook) and determine which treatments may be most helpful.

6. Immunohistochemistry Looks for HER2 and Other Proteins

A common method used by pathologists to study biopsy samples is called immunohistochemistry (IHC). This test, sometimes referred to as an assay, helps doctors find out whether cancer cells contain high levels of HER2 proteins or other important markers.

To perform an IHC test for HER2, the pathologist attaches a piece of the biopsy tissue to a glass slide. They then add a chemical solution containing an anti-HER2 antibody — a molecule that recognizes and binds to a specific protein. If the cells have the HER2 protein, the antibody attaches to it and causes the cells to change color.

The more HER2 protein is found on the cells, the darker the cells will appear under the microscope. The pathologist reports your IHC results using a score.

How Long Does IHC Take?

It usually takes several days to a couple of weeks to get IHC results. Different factors can affect the timing, such as how busy the lab is or whether additional testing is needed. Ask your doctor for an estimate of when you can expect your results.

What Do IHC Results Mean?

The pathologist reports your IHC results as a number:

  • 0 or 1+ — The cells have no or very little HER2, and the cancer is considered HER2-negative.
  • 2+ — The cells have some HER2, but it is not clear if the cancer should be considered negative or positive.
  • 3+ — The cells have a lot of HER2, and the cancer is considered HER2-positive.

IHC may also be used to test for other cancer-related proteins in your biopsy sample, including estrogen and progesterone receptors.

7. FISH Looks for HER2 Genes

If your IHC results for HER2 are 2+, also called “equivocal results,” you will probably need another laboratory test called fluorescence in situ hybridization (FISH).

In most cases, you won’t need another procedure. The pathologist can use the cells from your original biopsy sample. FISH measures the amount of HER2 genes inside the breast cancer cells.

If your IHC test results for HER2 are unclear, you will probably need another laboratory test called fluorescence in situ hybridization (FISH).

How Long Do FISH Results Take?

FISH results are usually available within a few days. This test targets DNA inside the cells directly, making it faster to get answers without needing to grow cells in the lab.

What Does a Positive FISH Test Result Mean?

FISH results are either positive or negative.

  • FISH-positive means the tumor is HER2-positive.
  • FISH-negative means the tumor is HER2-negative.

If you have questions about your FISH results, your healthcare team can explain what they mean for your treatment options.

IHC vs. FISH

While IHC tests for HER2 protein overexpression (higher-than-normal levels of the HER2 protein), FISH tests for HER2 gene amplification (extra copies of the HER2 gene inside the cells).

FISH results are often considered more accurate than IHC, but the test usually costs more and takes a little longer to process.

Chromogenic In Situ Hybridization

Most people with breast cancer will have IHC and/or FISH testing to determine their HER2 status. However, in some cases, another method called chromogenic in situ hybridization (CISH) may be used.

Like HER2 FISH testing, CISH uses pieces of DNA to measure HER2 overexpression. The more HER2 genes your cells have, the more the cells will change color during the test.

CISH is thought to be just as accurate as FISH while being less expensive. However, not as many laboratories use this method.

What Happens After Your HER2 Test?

Once you have your HER2 results, your doctor should talk to you about the next steps.

Breast cancers that are HER2-positive often respond very well to targeted therapies — medications that identify and attack cells with the HER2 protein. If you have HER2-positive breast cancer, your doctor will probably recommend that you use targeted therapies or a combination of treatment methods.

Read more about treatment options for HER2-positive breast cancer.

At some point after you begin receiving breast cancer treatment, your doctor may have you undergo more HER2 testing. Results can help show whether the treatment is working. If you were diagnosed with HER2-positive cancer, and later tests show that your HER2 levels are normal, this may mean your tumor is shrinking. High HER2 levels, on the other hand, may indicate that your cancer has come back.

Your doctor may recommend maintenance therapy after initial treatment. Maintenance therapy may involve ongoing treatment to lower the risk that breast cancer may return.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer. On MyBCTeam, members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Have you been diagnosed with HER2-positive breast cancer? What was your diagnostic process like? Share your experience in the comments below, or start a conversation by posting on your Activities page.

A MyBCTeam Member

I was diagnosed with Her2 positive estrogen. It was back in 2010. I had both breast removed for it was aggressive. When they removed my tumor it had 3 more clusters growingon top of it. So I started… read more