Metastatic breast cancer is cancer that has spread outside of your breast to other organs like your lung, brain, or liver. Your doctor may refer to this cancer as stage 4, or late-stage breast cancer.

Your healthcare team will do a number of tests to diagnose your breast cancer, see how far it has spread, and find the right treatment. Genetic tests are one part of the diagnosis process. These tests can tell your doctor whether your cancer is related to a genetic mutation and what treatment might work best.

Not everyone needs genetic testing. Your doctor and genetic counselor will recommend these tests based on your age and risks.

Genes are segments of DNA. They live inside the nucleus of each cell in your body. Genes carry the instructions for making the proteins that control all of your body’s activities.

Having certain gene changes, called mutations, can increase your likelihood of getting breast cancer. Genetic testing looks for these changes to individual genes. Gene tests also analyze chromosomes — large sections of DNA — to look for changes linked to breast cancer.

Your doctor may order tests to look for BRCA1, BRCA2, and HER2 gene mutations. Other gene tests are available, but they’re not used as often.

BRCA gene tests

BRCA1 and BRCA2 genes produce a type of protein known as tumor suppressor proteins. When these genes are normal, they fix damaged DNA and help to prevent cancer cells from growing.

Mutations in the BRCA1 and BRCA2 genes trigger excess cell growth and increase your risk for both breast and ovarian cancers.

A BRCA gene test can help your doctor learn your breast cancer risk. If you already have breast cancer, testing for this gene mutation can help your doctor predict whether certain breast cancer treatments will work for you.

HER2 gene tests

Human epidermal growth factor receptor 2 (HER2) codes for the production of the receptor protein HER2. This protein is on the surface of breast cells. When the HER2 protein is turned on, it tells breast cells to grow and divide.

A mutation in the HER2 gene puts too many HER2 receptors on breast cells. This causes breast cells to grow uncontrollably and form tumors.

Breast cancers that test positive for HER2 are called HER2-positive breast cancers. They grow faster and are more likely to spread than HER2-negative breast cancers.

Your doctor will use one of these two tests to check your HER2 status:

  • Immunohistochemistry (IHC) tests whether you have too much of the HER2 protein on your cancer cells. The IHC test gives the cancer a score of 0 to 3+ based on how much HER2 you have on your cancer. A score of 0 to 1+ is HER2-negative. A score of 2+ is borderline. And a score of 3+ is HER2-positive.
  • Fluorescence in situ hybridization (FISH) looks for extra copies of the HER2 gene. The results are also reported as HER2-positive or HER2-negative.

If you’ve been diagnosed with metastatic breast cancer, it may be helpful to learn whether an inherited mutation caused your cancer. Genetic testing can help guide your treatment. Certain cancer drugs only work or are more effective in breast cancers with specific gene mutations.

For example, the PARP inhibitor drugs olaparib (Lynparza) and talazoparib (Talzenna) are only FDA-approved to treat metastatic breast cancer caused by a BRCA gene mutation. People with these mutations may also respond better to the chemotherapy drug carboplatin than docetaxel.

Your gene status may also help determine what type of surgery you get and whether you’re eligible to join certain clinical trials. It can also help your children or other close relatives learn whether they might be at higher risk for breast cancer and need extra screening.

Guidelines from the National Comprehensive Cancer Network recommend genetic testing for people with breast cancer who:

  • were diagnosed at or before age 50
  • have triple-negative breast cancer that was diagnosed at or before age 60
  • have a close relative with breast, ovarian, prostate, or pancreatic cancer
  • have cancer in both breasts
  • are of Eastern European Jewish descent (Ashkenazi)

However, a 2019 guideline from the American Society of Breast Surgeons recommends that all people who are diagnosed with breast cancer be offered genetic testing. Talk to your doctor about whether you should get tested.

For the BRCA gene tests, your doctor or nurse will take a sample of your blood or a swab of saliva from the inside of your cheek. The blood or saliva sample then goes to a lab, where technicians test it for the BRCA gene mutations.

Your doctor performs HER2 gene tests on breast cells removed during a biopsy. There are three ways to do a biopsy:

  • Fine needle aspiration biopsy removes cells and fluid with a very thin needle.
  • Core needle biopsy removes a small sample of breast tissue with a larger, hollow needle.
  • Surgical biopsy makes a small cut in the breast during a surgical procedure and removes a piece of tissue.

You and your doctor will get a copy of the results, which come in the form of a pathology report. This report includes information on the type, size, shape, and appearance of your cancer cells, and how quickly they’re likely to grow. The results can help guide your treatment.

A genetic counselor is a specialist in genetic testing. They can help you decide whether you need genetic tests and the benefits and risks of testing.

Once your test results are in, the genetic counselor can help you understand what they mean, and what steps to take next. They can also help inform your close relatives about their cancer risks.

If you’ve been diagnosed with metastatic breast cancer, talk to your doctor about genetic testing. It may help to speak with a genetic counselor to understand what your tests mean.

The results of your genetic tests can help your doctor find the right treatment for you. Your results can also inform other members of your family about their risk and need for extra breast cancer screening.