HER2 FISH Testing for Breast Cancer: What to Know

Medically Reviewed by Melinda Ratini, MS, DO on May 25, 2022
3 min read

When you get a diagnosis of breast cancer, your doctor will use several tests to learn details about your tumor and the best way to treat it. The FISH (fluorescence in situ hybridization) test checks the DNA of your cancer cells for extra copies of the HER2/neu gene.

This gene makes proteins called HER2 (human epidermal growth factor receptor 2) that attach to the surface of all breast cells. In healthy tissue, the proteins help cells grow, divide, and do repairs. If your breast cancer cells have too many copies of this gene, they make too much of the protein. This can make your tumor grow faster.

FISH testing can tell your doctor whether you have this type of cancer, called HER2-positive breast cancer, or a HER2-negative cancer with normal levels of the protein.

At the time of a breast cancer diagnosis, FISH test results can help your doctor choose the treatments that will work best for you. There are several kinds of drugs that target and lower HER2 levels. These treatments work well for cancers that make too much of the protein. If your test finds high levels, your doctor may recommend one or more of these medications.

If your protein levels are normal, drugs that target HER2 aren’t useful for your cancer, so you’ll need different medications.

Your doctor can use more than one method to check your HER2 levels. If your results weren’t clear with the first test, your doctor may also do or repeat a FISH test to double-check them.

Your HER2 levels may change if your cancer comes back after treatment or spreads beyond the breast tissue. If this happens, your doctor may do a FISH test to understand which treatments will now work best for you.

Doctors use a sample of breast cancer tissue to do FISH testing.

As part of your diagnosis, your doctor will take a small amount of breast cancer tissue in a procedure called a biopsy. The sample goes to a lab, where doctors will use some of it for FISH testing.

Most of the time, doctors can do your breast cancer biopsy with one of several types of hollow needles, which they use to draw out the tissue. Your doctor will first give you a shot to numb the area and then insert the needle and withdraw the sample.

Your tumor’s location, size, or another factor may mean you need a surgical biopsy. If you do, your doctor may give you medication to make you drowsy and then follow with a shot to numb the area, or they may use general anesthesia that will make you sleep so you won’t feel pain. Then your doctor will cut into your skin and remove the tissue.

FISH testing usually returns one of two results: positive or negative.

Positive means your breast cancer cells make too much HER2 and your doctor should treat you with drugs that target that protein.

Negative means the protein isn’t involved in the growth of your tumor. If you get this result, your doctor will choose a different treatment plan.

Sometimes FISH test results are unclear. If this happens, your doctor may do more tests to try to get a clear answer about your HER2 protein levels.