FISH tests

A FISH test is a lab test that can confirm breast cancer’s HER2 status if other test results are inconclusive. The test is performed on tissue taken from a biopsy or breast cancer surgery.

FISH stands for fluorescence in situ hybridization. In breast cancer, a FISH test uses a fluorescent dye to find extra copies of the HER2 gene.

Too many copies of the HER2 gene produces too many breast cancer HER2 receptors. HER2 receptors are biomarkers that signal the cancer to grow and spread more quickly beyond the breast.

FISH results can tell if breast cancer is HER2-positive, HER2-low, or HER2-negative. This can help you and your healthcare team make treatment decisions.

Below, we’ll explain more about when FISH testing may be recommended, how it works, and what the results mean.

Who gets FISH testing?

Everyone diagnosed with breast cancer should have HER2 testing, but not everyone needs a FISH test. Often the first HER2 test is an immunohistochemistry (IHC) test. IHC tests are typically less expensive and done more quickly than FISH tests.

In many cases, FISH testing is performed only if the result of an IHC test is not clearly HER2-positive or HER2-negative. If an IHC test comes back with a 2+/equivocal result, then FISH testing is automatically performed next. This is done at the lab without needing any more tissue from you.

HER2 testing is especially important to reconfirm HER2 status in people who had early breast cancer come back as metastatic breast cancer. This is because it is possible for the HER2 status to change over time.

How does FISH testing work?

If initial IHC testing for HER2 has a result of 2+, this result is not strong enough to confirm the HER2 status. In this case, a FISH test will automatically be used for clarification.

A FISH test is one example of an in situ hybridization (ISH) test. ISH tests use chemicals that attach to DNA. This helps specially trained doctors, called pathologists, find and count specific genes.

In FISH tests for HER2 status, the chemical is a fluorescent dye that lights up when it connects with HER2 genes. A pathologist counts the HER2 genes to figure out if the cancer is HER2-positive or HER2-negative.

How FISH test results are reported

FISH test results are reported as positive or negative. But many pathology reports have no FISH score. This is because HER2 status is often confirmed with an IHC test score.

Still, sometimes an IHC score is not enough. An IHC score of 2+ (sometimes reported along with the word “equivocal”) will signal the lab to conduct a FISH test to verify the cancer’s HER2 status.

If this happens, the pathology report will list the IHC score, the FISH results, and a final HER2 status.

The report may also contain some of the findings that led to the decision, such as the number of HER2 genes.

In metastatic breast cancer, an IHC score of 1+ or an IHC score of 2+ followed by a negative FISH test may be referred to as HER2-low. HER2-low is not a term that is listed on pathology reports right now, but your doctor should tell you if the cancer is HER2-low.

How FISH results can guide treatment options

A FISH test is one of many tests that can help you and your doctor make treatment decisions. If a FISH test is needed, it can help confirm the following breast cancer subtypes and possible treatments:

• HER2-positive: The cancer can be treated with HER2-targeted therapies such as trastuzumab (Herceptin). Learn more on the HER2-positive or HER2-low breast cancer page.
• HER2-negative: Although HER2-targeted therapies won’t be effective, other features of the cancer, such as hormone receptor status, can be used to make treatment choices.
• HER2-low: If the breast cancer is metastatic and HER2-low, trastuzumab deruxtecan may be an option. You can also ask your doctor about clinical trials studying HER2-low. Visit HER2-positive or HER2-low breast cancer to learn more.
• Triple-negative: If the cancer is found to be HER2-negative and is also negative for hormone receptors, it is triple-negative. This cancer subtype may not respond to therapies used in other breast cancer types. But many effective treatments, such as immunotherapy and targeted therapies, are available. Read triple-negative breast cancer for more information.
• In some cases of metastatic breast cancer, FISH testing is used to look for a genetic change called NTRK gene fusion. There are two medicines that target NTRK gene fusion: larotrectinib (Vitrakvi) and entrectinib (Rozlytrek). Read more about NTRK gene fusion on the Biomarker testing page.

Whatever the results of your FISH test, your doctors are ready with effective treatment options. As you learn more about your diagnosis, LBBC is here with trusted information and resources for you.