The cancer antigen 15-3 (CA 15-3) test is a blood test that can indicate how a person is responding to treatment for invasive breast cancer or check for the recurrence of breast cancer.

Healthy cells in the breast release a protein called CA 15-3. When a person has breast cancer, cancerous cells or tumors in the breast can release higher levels of CA 15-3 into the bloodstream.

This article explains when doctors may use the test and what the results can show. It also looks at the role the test can play in the diagnosis and treatment of breast cancer.

A syringe that is used during a CA15-3 blood test for breast cancer.Share on Pinterest
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A CA 15-3 test is a blood test that checks for CA 15-3 levels in the blood. CA 15-3 is a protein that breast cells produce normally.

Doctors may refer to the CA 15-3 protein as a tumor marker. This term refers to any substance that cancer cells — or other cells that are responding to cancer — contain or produce. In some cases, certain benign conditions can also lead to the production of tumor markers.

When there are cancerous cells, or tumors, in the breast, the levels of CA 15-3 can increase. Cancerous breast tumors shed higher-than-normal levels of CA 15-3, which then enters the bloodstream.

Doctors may also refer to the test as a cancer antigen breast test or a CA breast test.

A person may have a CA 15-3 test if they are currently undergoing treatment, or have had treatment, for invasive breast cancer.

The test can help a healthcare professional determine how well treatment is working or check for breast cancer recurrence. A CA 15-3 test can be a useful test to show whether a tumor is spreading or decreasing in response to treatment.

Doctors may also recommend a CA 15-3 test for people who have received a diagnosis of advanced breast cancer.

According to Lab Tests Online, CA 15-3 levels are elevated in about 80% of people with breast cancer that has spread to other areas of the body.

The test is not as useful for people with early stage or localized breast cancer, as less than 50% of these cases will involve elevated CA 15-3 levels.

Are there alternative tests?

Some people may have a CA 27.29 test instead of a CA 15-3 test. Both tests carry out the same function and measure the same protein.

The CA 15-3 test is a blood test that can take place at a doctor’s office or in a hospital. A healthcare professional will use a needle to take a sample of blood from a vein in the arm.

They will then send the blood sample to a laboratory for testing. Depending on the laboratory, people may have to wait several days for the results.

Does a person need to prepare for the test?

There is no need to prepare for a CA 15-3 test, but a person should make their doctor aware of any medications, supplements, or herbal remedies they are taking. A doctor may advise stopping taking some substances a few days before any lab tests.

For instance, according to the Food and Drug Administration (FDA), biotin supplements — which some people take to promote the growth of hair and nails — may interfere with certain lab tests.

The daily recommended biotin allowance for adults, which is 0.03 milligrams (mg), is unlikely to cause any issues. However, the United Kingdom’s National Health Service (NHS) suggests that healthcare professionals wait 8 hours before collecting the sample if a person is taking more than 5 mg per day of biotin.

A normal test result for CA 15-3 levels is usually 30 units per milliliter (U/ml) or less.

Lab Tests Online notes that the levels of CA 15-3 generally increase in line with the advancement of breast cancer. Increased levels of CA 15-3 may, therefore, indicate that the cancer has spread to other areas of the body.

CA 15-3 levels that continue to increase over time may indicate that the:

  • treatment is not working effectively
  • cancer is continuing to spread
  • cancer is recurring

Decreasing levels of CA 15-3, or levels that return to normal, may indicate that the treatment is working effectively.

In some cases, a person with normal CA 15-3 levels may still have breast cancer. In the early stages of breast cancer, CA 15-3 levels may not be high enough for a test to detect.

Also, about 20–25% of people with advanced breast cancer have tumors that do not release CA 15-3.

Additional tests can help support the findings of a CA 15-3 test and confirm how cancer is affecting the body.

Some laboratories use different methods to test for CA 15-3, which can produce varying results.

If a person needs multiple CA 15-3 tests over time, they should ask for the tests to take place in the same laboratory using the same method. This ensures that doctors can interpret and compare the results correctly.

It is important to note that not all doctors monitor the levels of CA 15-3, as other tumor markers are more sensitive. A 2018 study found that the tissue polypeptide-specific (TPS) antigen was the best candidate for predicting the recurrence of breast cancer.

In addition, the CA 15-3 test is not always a reliable indicator of breast cancer, or recurring breast cancer. Generally, increasing levels of CA 15-3 indicate that breast cancer is advancing or recurring, while decreasing levels suggest that treatment is working effectively to reduce cancer. However, other factors, such as ethnicity and other medical conditions, can affect the levels of CA 15-3.

Research from 2020 that focused on females without breast cancer found that lower levels of CA 15-3 were present in those who:

  • are perimenopausal
  • have a history of uterine fibroids
  • have taken birth control pills for an extended period

The researchers also found that elevated levels of CA 15-3 were present in Hispanic and Black females, as well as those with a history of endometriosis or lung cancer.

Doctors will use other tests to confirm any findings from a CA 15-3 test in order to make an accurate diagnosis or assess how the treatment is working.

There are minimal risks to the CA 15-3 test, but people may experience the following at the needle insertion point:

  • bleeding
  • bruising
  • infection
  • mild stinging or pain when the needle enters
  • soreness

Some people may feel lightheaded after the test. A healthcare professional will check that a person is feeling well before they leave. However, it is important to contact a doctor if there are any signs of infection or severe symptoms afterward.

According to a 2016 article, tumor markers may not be accurate indicators, and false-positive results can lead to emotional distress and fear.

A CA 15-3 test is a blood test that doctors may use for people with a breast cancer diagnosis and those who have had breast cancer. The CA 15-3 test shows the levels of CA 15-3 protein in the blood, which can increase as breast cancer advances.

Decreasing levels of CA 15-3 may indicate that treatment is working effectively to reduce the spread of cancer. However, CA 15-3 can sometimes indicate other health issues, such as liver disease or benign conditions of the breast. Doctors will, therefore, likely use other tests to confirm the findings.