​Most people with breast cancer will undergo tests to determine whether their cancer is HER2-positive or HER2-negative. It’s important to find out this information as early as possible because your HER2 status tells you and your doctor which treatments are most likely to work.

Up to 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 many copies of the HER2 gene (also called human epidermal growth factor receptor 2, HER2/neu, or ERBB2).

Read more about the HER2 gene, HER2 protein, and breast cancer prognosis.

1. It Begins With Breast Cancer Diagnosis

Receiving a breast cancer diagnosis often begins with discovering a mass or lump. This may happen during a doctor’s appointment, a breast self-examination, or a routine mammogram screening for breast cancer.

Imaging tests, such as an ultrasound or MRI, may be used to further examine the lump, but they can’t diagnose whether a mass is cancerous.

2. Biopsy Is the First Step in HER2 Testing

A biopsy is the first step in both breast cancer diagnosis and HER2 testing. During a biopsy, your doctor will take a small sample of tissue from your breast for microscopic study. A biopsy helps the doctor tell whether you have cancer and, if so, what type and how advanced it is. Biopsy tissue can also show which genetic changes are in your cancer cells, which can help the doctor determine the best treatment plan.

3. There Are Different Types of Biopsies

There are a few ways a doctor can remove a sample of cells. They may recommend a specific type of biopsy based on factors like how large the mass is, its location within the breast, and your personal preferences.

Needle-Based Biopsy

If your doctor suspects breast cancer, they will probably recommend a core needle biopsy. During this procedure, the doctor will numb part of the breast, make a small cut into your skin, and remove multiple small cylinder-shaped pieces of breast tissue using a hollow needle.

Another possible biopsy option is a fine-needle aspiration (FNA). This procedure also involves using a needle, but the needle is generally thinner and removes fewer cells. You may not receive any numbing medication for FNA because it is usually quick and doesn’t hurt very much (just like 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 the biopsy, the doctor may put a dressing over the area. The affected area usually doesn’t take too long to heal, but there may be some swelling or bleeding at first. Follow your doctor’s or nurse’s instructions to take care of the biopsy area.

Surgical Biopsy

Surgical breast biopsies aren’t as common as core needle biopsies or FNAs. They may be used when a needle biopsy can’t reach the affected tissue or if the results of a needle biopsy are unclear.

During a surgical breast biopsy, a doctor will remove all or part of the breast lump. The procedure usually takes place in a hospital, and you will probably be given medicine to make you sleepy and help you not feel the pain.

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

In most cases, a biopsy will show that there is no cancer. However, if you do have cancer, the biopsy will give the doctor more information about what type of cancer cells are present. A biopsy can also help the doctor find out how advanced your breast cancer is and assign it a stage and grade.

After your biopsy sample is taken, it will be sent to a laboratory, where it will be studied by a doctor called a pathologist. It usually takes a few days to get the biopsy results back. Your doctor can give you a better idea of when results may be available and how you can see them. The pathologist may look for several factors, including proteins such as HER2.

5. Cancer Cells Are Graded Based on Microscope Study

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. Cancer cells are more likely to have unusual shapes and sizes. Additionally, normal cells are usually arranged in a neat order. Cancer cells, on the other hand, are disorganized and form unusual-looking structures. They can start growing in nearby tissues. Looking for these features helps pathologists recognize whether the cells in the biopsy sample are normal or cancerous.

A pathologist can assign a breast cancer grade based on how the cells appear. When cancer cells don’t look too unusual, they are lower grade, meaning they are expected to grow more slowly. When the cancer cells look very different from normal breast cells, they are higher grade. Your doctor can use information about your cancer’s grade along with your HER2 status as predictive factors to estimate your 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 an immunohistochemical stain. This test, known as an assay, can help doctors tell whether your cancer cells contain high levels of HER2 hormone receptors or other proteins.

To perform this HER2 test, the pathologist will attach some of the tissue samples from the biopsy to a glass slide. They will then add a mixture of chemicals containing an antibody. An antibody is a molecule that can recognize and attach to a specific protein. If the cells contain this protein, the antibody will attach and the cells will change color. By using an anti-HER2 antibody that recognizes the HER2 protein, a pathologist can see how much of this protein the breast cancer cells have.

The more HER2 protein is found on the cells, the darker the cells will look. The pathologist will report your immunohistochemistry (IHC) results as a number.

How Long Does IHC Take?

IHC results may take several days to weeks to be available. Many different factors can affect the amount of time it takes to get these test results. For an estimate, ask your doctor when you can expect to receive your IHC results.

What Do IHC Results Mean?

• A result of zero or 1-plus means the cells have no or very little HER2, and the cancer is considered HER2-negative.
• A result of 2-plus shows the cells have some HER2, but it is not clear if the cancer should be considered negative or positive.
• A result of 3-plus means 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. Fluorescence In Situ Hybridization Looks for HER2 Genes

If your IHC results for HER2 are 2-plus, also called “equivocal results,” you will probably need another laboratory test called fluorescence in situ hybridization (FISH). In this case, you usually won’t need to have additional procedures. The pathologist can use cells from your original biopsy sample. FISH also measures the amount of HER2 within your breast cancer cells.

How Long Does FISH Take?

FISH results are usually available within a few days. FISH targets DNA in cells directly, making it easier and faster to diagnose cancer without needing to grow cells in lab dishes.

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 how to read FISH results, your health care provider can help.

IHC vs. FISH

While IHC tests for protein overexpression (high levels of the HER2 protein), FISH looks for HER2 gene amplification (lots of copies of the HER2 gene). FISH results may be more accurate than IHC results, but FISH costs more and takes longer to process.

Chromogenic In Situ Hybridization

Most people with breast cancer will have IHC and/or FISH to determine whether their cancer is HER2-positive or HER2-negative. However, in some cases, other types of in situ hybridization may be used.

Chromogenic in situ hybridization (CISH) is occasionally used instead of FISH. 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, but it may be 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.

Read about ways to stay healthy after HER2-positive breast cancer treatment.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer. On MyBCTeam, more than 69,000 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.