What to Know About the Immunohistochemistry Test

Medically Reviewed by Melinda Ratini, MS, DO on May 16, 2024
5 min read

The immunohistochemistry (IHC) test can tell your cancer doctor (oncologist) whether you have certain types of breast cancer. Professionals in a lab check a sample of your cancer cells. Then your doctor can use the results to figure out the right treatment for you.

The IHC test is the most common way for your doctor to find out if your breast cancer is fueled by hormones or by a protein called HER2.

Most breast cancers grow in response to estrogen. The hormone progesterone can also drive growth. So can a combination of both hormones. Doctors call these types of breast cancer “hormone receptor-positive.”

Receptors are proteins in or on cells that can attach to certain substances in your blood. Some breast cancer cells have receptors that attach to estrogen, progesterone, or both. The American Cancer Society says all breast cancers should be tested for hormone receptors.

The IHC test can also let your doctor know if your breast cancer has too many receptors for a protein called HER2, which can fuel the growth of cancer cells as well. Up to 20% of breast cancers are this type, called “HER2 positive.” The American Cancer Society says all newly diagnosed breast cancers should be tested for HER2.

A lab tests a sample of your breast cancer cells. Your doctor collects this sample during a biopsy to diagnose breast cancer or during surgery to help treat your cancer.

When professionals at the lab test your sample, they use specific lab-made proteins called antibodies. These antibodies can detect spots on receptors for HER2 or for the hormones estrogen and progesterone and attach to them.

If your breast cancer cells have any of these receptors, the antibodies stick to them -- and chemicals added to the antibodies receptor complex cause the cells to change color. Lab professionals can then see the color change under a microscope.

Ask your doctor to go over your results with you and clearly explain everything.

If you get an IHC test for HER2 receptors, the lab gives your doctor a number for a result:

  • 0 or 1+ means your breast cancer doesn’t have receptors for HER2. Your doctor calls this HER2-negative.
  • 2+ means it’s unclear whether your cancer is fueled by HER2. Your doctor may call this result “equivocal” or “borderline.”
  • 3+ means your breast cancer has HER2 receptors. Your doctor calls this HER2-positive.

If your IHC test result is 2+ (borderline), you’ll need a follow-up exam called the fluorescence in situ hybridization (FISH) test to get a clearer result. Some experts say you should also consider getting the FISH test if your IHC result is 0 or 1+ (HER2-negative). While lots of breast cancer specialists think the FISH test is more accurate than the IHC, it’s more expensive and takes longer to deliver results.

Research shows that HER2 test results can be wrong. That’s part of the reason some experts recommend you get re-tested in certain situations. They say you should also consider getting your results reviewed by more than one lab professional (“pathologist”).

If you get an IHC test for estrogen and progesterone receptors, your lab may present the results in one of few different ways (depending on the reporting method it uses):

  • A percentage (between 0% to 100%) of how many of your breast cancer cells out of 100 have hormone receptors
  • A number from 0 to 3: Zero means the test found no receptors, 1 means it found a small number of them, 2 means a medium number, and 3 means a large number.
  • An “Allred score” between 0 and 8: The higher the score, the more receptors the test spotted and the easier they were to see after changing colors under the microscope.
  • The word “positive” (meaning the test spotted hormone receptors) or “negative” (meaning it didn’t find them)

If you get a result of “positive” or “negative,” some experts recommend that you ask your doctor for a more clear-cut percentage, rating, or other figure. That’s because different labs have different standards for labeling a breast cancer hormone receptor-positive or hormone receptor-negative. If your result says “negative,” ask your doctor to fully explain why. Also consider asking them to have the lab re-do the test.

Some experts also suggest that you ask your doctor to send your sample to a lab with lots of experience in IHC testing for hormone receptors. That could boost the odds that you’ll get a more accurate result back.

The results can help your cancer doctor figure out the best treatment for you:

HER2-positive. This type of breast cancer can be treated with drugs that target HER2.

HER2-negative. This type grows more slowly and is less likely to come back compared to HER2-positive. It doesn’t respond to HER2-targeting drugs, though, so your doctor will choose a different treatment for you. For example, if you have early breast cancer that’s HER-2 negative and hormone-receptor positive, your treatment may include hormone-therapy drugs and chemotherapy.

Hormone receptor-positive. This form of breast cancer can be treated with anti-estrogen hormone-therapy drugs that lower your estrogen levels or block or destroy estrogen receptors. Certain types of targeted therapy meds can boost how well the hormone therapy works, but they may give you more side effects.

Hormone receptor-negative. This type of cancer usually grows faster than hormone receptor-positive, and hormone therapy drugs won’t be helpful to you. Your doctor will choose a different treatment that works better.

Triple-positive breast cancer. This type has receptors for estrogen, progesterone, and HER2. Hormone drugs and meds that target HER2 can treat it.

Triple-negative breast cancer. This type doesn’t have estrogen or progesterone receptors, and it also doesn’t make any, or a lot, of HER2. It usually grows and spreads faster than most other kinds of breast cancer. Hormone therapy meds and drugs that target HER2 don’t help treat it. But your options may include surgery, chemotherapy, and radiation.