Health Encyclopedia

CA 15-3

Does this test have other names?

Cancer antigen 15-3

What is this test?

This test measures the amount of CA 15-3 in your blood. CA 15-3 is an antigen, or a substance that stimulates your body's defense system. Some kinds of cancer cells release the CA 15-3 antigen into the blood. This test is used to monitor certain types of cancer.

Breast cancer is the cancer most likely to release CA 15-3, especially in breast cancer that comes back after treatment. Antigens like CA 15-3 that give information about cancer are called tumor markers.

Why do I need this test?

You may need this test to monitor your body's response to treatment for breast cancer. CA 15-3 is not used as a screening test or a diagnostic test to find breast cancer. Some people who have breast cancer do not have high levels of CA 15-3. Also, conditions other than breast cancer can cause a positive CA 15-3 test.

You may need this test if:

  • You have already been diagnosed with breast cancer and your doctor wants to find out how well treatment is working.

  • You have already been treated for breast cancer and your doctor wants to find out if your cancer has come back or spread.

What other tests might I have along with this test?

Your doctor may order a test to check for another breast cancer tumor marker called CA 27-29. You may also have other diagnostic tests to learn about your cancer.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

CA 15-3 is measured in units per milliliter (U/mL). A normal test should be less than or equal to 30 U/mL.

It is important to know that having a positive CA 15-3 blood test does not mean you have breast cancer or that your breast cancer has come back. Other conditions can cause CA 15-3 to be in your blood.

Noncancerous conditions of the breast, ovary, and liver can cause CA 15-3 to go up.

Many factors influence test results and the results of this test are evaluated with other clinical information. In general, increasing values may indicate disease progression or recurrence. Decreasing values may indicate disease regression.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, and a sense of lightheadedness. When the needle is put in your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be sore.

What might affect my test results?

Other factors aren't likely to affect your results.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use. 



Medical Reviewers:

  • Fraser, Marianne, MSN, RN
  • Marcellin, Lindsay, MD