doctor discussing her2 test results

When you’re diagnosed with breast cancer, you’re probably most concerned about getting rid of the cancer, not finding out more about it. But the more you know about what type of cancer you have, the better treatment you’ll likely receive.

HER2-positive is one type of breast cancer. Up to 20 percent of women with breast cancer have tumors with high levels of HER2, a protein/receptor that makes cancer cells grow and spread faster.

Should You Be Tested for HER2?

The American Society of Clinical Oncology recommends that all people diagnosed with invasive breast cancer be tested for HER2. Invasive breast cancer is cancer that starts in the milk ducts or glands of the breast but grows into surrounding breast tissue. Most breast cancers are invasive.

It’s important to know whether your cancer is HER2-positive or HER2-negative. It will help your oncology team decide which type of breast cancer treatment is best for you.

HER2-positive tumors tend to spread more aggressively than other breast cancers, but there are life-saving medicines that target HER2. Most women with breast cancer will receive a combination of different therapies.

Several studies show that HER2-targeted therapies can improve the odds of survival in women with HER2-positive breast cancer.

If you have recurrent breast cancer, or breast cancer that comes back after treatment, your oncology team may also test you for HER2, even if you were previously diagnosed with HER2-negative cancer.

There are a number of reasons to retest for HER2. First, it’s possible that a previous HER2 test could’ve been wrong. Second, the test for your original breast cancer could have been borderline for HER2. Third, when breast cancer comes back, it can change from HER2-negative to HER2-positive. It’s also important to know that within one tumor, tissue from one area can test HER2-positive and tissue from another part may be HER2-negative.

If you have ductal carcinoma in situ (DCIS), a form of noninvasive or pre-invasive breast cancer, you’ll not be tested for HER2. DCIS isn’t currently treated with these drugs, but studies are ongoing.

There are two main tests for HER2: immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH).

IHC Test for HER2

The IHC test is often the first test used to determine HER2 status. This test finds out if there are too many HER2 receptors on the surface of the cancer cells.

In this test, special proteins are used that can find and attach to HER2. If there’s too much HER2, the cancer cells will change color. Your oncology team will look for this color change under a microscope.

IHC test results include:

  • 0 (HER2-negative)
  • 1+ (HER2-negative)
  • 2+ (HER2 borderline)
  • 3+ (HER2-positive)

If your IHC test shows a 0 or a 1+, your breast cancer is HER2-negative, which means it won’t be treated with those therapies. If your test is 3+, that means your breast cancer tumor is HER2-positive and your oncology team may recommend HER2 targeted therapies.

A 2+ result means that it’s not clear whether or not the cancer is HER2-positive or HER2-negative. If the results of the IHC test are unclear, your doctor will usually perform an additional test, called FISH.

FISH Test for HER2

FISH testing finds out whether there are too many copies of the HER2 gene in the cancer cells. The HER2 gene is responsible for making HER2 protein. The test results are reported as a ratio of gene copies to receptors on the outside of the cell.

In this test, fluorescent pieces of DNA stick to copies of the HER2 gene in the cancer cells. Your doctor will be able to count the copies under a special microscope that uses fluorescence.

The results of a FISH test will be either positive or negative. If the FISH test is positive, it means there are too many copies of the HER2 gene and you have HER2-positive breast cancer. Some breast cancer specialists think that FISH is more accurate than IHC. However, FISH is more expensive and it takes longer to get the results, so many doctors will use IHC first.

There are three other tests that can be used to find out whether there are too many copies of the HER2 gene: chromogenic in situ hybridization (CISH), silver-enhanced in situ hybridization (SISH), and dual in situ hybridization (DISH). These tests are used less commonly than FISH.

If you’ve just been diagnosed with breast cancer, ask your doctor or oncology team about HER2 testing.