Other than skin cancer, breast cancer is the most common cancer in people assigned female at birth. In fact, it makes up about 30 percent of all new diagnoses in this group each year, according to the American Cancer Society.

Some breast cancers are HER2-positive. This means that the cancer cells have high levels of HER2 protein (human epidermal growth factor receptor 2) on their surface, which helps them grow faster. From 2012 to 2016, about 15 percent of breast cancers were HER2-positive.

If you’ve been diagnosed with breast cancer, an IHC test can tell you if your cancer is HER2-positive. Keep reading to learn more about this test, when it’s used, and how it’s done.

IHC stands for immunohistochemistry. IHC testing is done on a sample of breast tissue that’s been collected through a biopsy or through surgery.

In an IHC test, antibodies that are specific to the HER2 protein are added to the tissue sample. If HER2 is present on the cells, these antibodies will stick to it.

When a special chemical is added to the sample, a color change will be seen in areas where HER2 is present. The magnitude of this color change is used when reporting your results.

The IHC test for HER2 determines the amount of HER2 protein present on the surface of cells collected through a biopsy or surgery.

Your doctor will also use the results from a biopsy or surgery to determine the clinical stage of the cancer. Staging for breast cancer also takes into account certain markers on the surface of cancer cells. These include hormone receptors and HER2.

When you’ve been diagnosed with breast cancer, it’s important to know the stage of your cancer, as well as your hormone receptor status and your HER2 status. This information helps determine the type of treatment you receive.

In the case of HER2-positive cancer, there are targeted therapy drugs that can block the activity of HER2. Examples include trastuzumab (Herceptin) and ado-trastuzumab emtansine (Kadcyla).

IHC testing is usually done on a sample of breast tissue that’s been collected through a biopsy. An IHC test can also be performed on tissue that’s been removed from a surgery, for example from a mastectomy or a lumpectomy.

When conducting a biopsy, doctors typically use imaging technology to guide a thin, hollow needle to the affected area where they then remove a tissue sample.

Prior to collecting the sample, you’ll be given an injection of a local anesthetic to numb the area where the needle will be inserted. While you may feel some pressure or discomfort during the procedure, you shouldn’t feel pain.

A breast biopsy is an outpatient procedure, meaning you go home when it’s over. The tissue sample will be sent to a pathologist for IHC testing. A pathologist is a doctor who specializes in analyzing tissue samples.

When the testing on your biopsy sample is complete, your doctor will contact you with the results. This typically happens a few days to a week after your biopsy.

The results of an IHC test are reported as a number value 0 through 3:

0 or 1+Your cancer is considered HER2-negative.
2+The HER2 status of your cancer is considered to be equivocal. Additional testing will be needed to more firmly determine its HER2 status.
3+Your cancer is considered HER2-positive.

If you receive a result of 2+ (equivocal), a fluorescence in situ hybridization (FISH) test is typically done as a follow-up.

FISH testing

FISH testing is generally considered to be more accurate than an IHC test. This is because IHC tests look for the amount of HER2 protein on the cell surface, while FISH tests look at the number of copies of HER2 genes in the cell.

However, FISH tests are more expensive and it can also take longer to get results, which is why IHC testing is often done first.

A FISH test uses a piece of DNA that has a fluorescent molecule attached. This piece of DNA can bind to the HER2 gene, HER2/neu, present inside of cells in the biopsy sample.

After the DNA piece has bound to HER2/neu, the attached fluorescent molecule allows a pathologist to count any extra copies of the gene under a microscope. The presence of extra copies of HER2/neu indicates HER2-positive cancer.

The results of an IHC test are based on the magnitude of the color change in the tissue sample after testing is completed. However, each lab or pathologist may have different ways of interpreting these changes.

Some tumors can also have a combination of cells that are HER2-positive and others that are HER2-negative, complicating the interpretation of IHC results. This is more common for tumors where HER2 status is found to be equivocal.

Because of these factors, it’s possible for an IHC test to be inaccurate, especially if the results are equivocal. This is one of the reasons FISH testing is often used as a follow-up.

Receiving an inaccurate IHC result can have implications on your breast cancer treatment:

  • A false-negative result can mean that your doctor likely won’t prescribe targeted therapy drugs directed at HER2, which can greatly benefit people with HER2-positive breast cancer.
  • A false-positive result can mean that you may receive targeted therapy drugs that aren’t effective for your breast cancer. You may also experience unnecessary side effects from these treatments.

If you’re concerned about the results of your IHC test, ask your doctor about doing a FISH test as well.

According to a 2020 report, the cost of an IHC test for breast cancer was $127 to $129 per sample. This may also vary according to factors like your healthcare team, your location, and the lab analyzing the sample.

These costs don’t include the cost of the biopsy or surgery that’s done to collect the sample used for an IHC test. However, most insurance providers will cover tests that are medically necessary for the diagnosis or treatment of a health condition, such as breast cancer.

If you’re unsure what’s covered, contact your insurance provider directly.

An IHC test is used to determine whether your breast cancer is HER2-positive. The results can have important implications on the type of treatment that’s recommended.

IHC testing is done on a tissue sample from a breast biopsy and uses antibodies that are specific for HER2 protein. If HER2 is present at high levels in the sample, a color change will happen when a special chemical is added.

It’s possible for IHC tests to be inaccurate. When there are concerns about the results of an IHC test, another test called a FISH test is used as a follow-up. This test is more accurate, although it may take longer to get your results.