This page was reviewed under our medical and editorial policy by
Bradford Tan, MD, Chair, Department of Pathology and Laboratory Medicine, City of Hope Atlanta, Chicago and Phoenix
This page was updated on December 7, 2022.
Cancer antigen 15-3 (CA 15-3) is a protein produced by cells in response to changes occurring in the body. CA 15-3 is considered a tumor marker because high levels of the antigen may indicate certain cancers, including breast cancer. CA 15-3 tests aren’t used to diagnose cancer, but rather to assess how cancer is responding to treatment. CA 15-3 fluctuations help cancer doctors determine whether a tumor is stable, growing, shrinking or has recurred.
Cancerous cells and tumors release CA 15-3 into the blood. The amount of CA 15-3 may be measured with a simple blood test. The CA 15-3 test isn’t used to diagnose breast cancer because early-stage or localized breast cancer doesn’t generally cause CA 15-3 levels to rise. Other diagnostic tests are used to evaluate patients at the pre-diagnostic stage of disease.
Instead, the CA 15-3 test is used to track the progression of advanced-stage cancer and how it responds to treatment. The test, part of a comprehensive cancer treatment plan, is used in conjunction with other gene, protein and blood tests.
A patient may need a CA 15-3 blood test if:
A CA 15-3 test is administered in a cancer center or other health care facility, where a medical professional uses a small needle to collect a sample of blood from a vein in the arm to be sent to the lab for testing.
The process is generally quick and painless, though patients may feel a brief pinch as the needle is inserted. Because some medications or supplements may affect CA 15-3 test results, patients should discuss these with their doctor prior to taking the test.
CA 15-3 is measured in units per milliliter or U/mL. The normal range for a healthy person is 30 U/mL or less. Further testing is recommended if CA 15-3 levels are above the normal range. According to the National Institutes of Health, CA 15-3 levels are elevated in approximately 76 percent of metastatic breast cancer cases.
In addition to breast cancer, lung cancer and ovarian cancer are also linked to increased CA 15-3 levels. Keep in mind, the presence of CA 15-3 doesn’t necessarily mean the patient has breast cancer or that cancer has recurred. Benign conditions of the breast, ovary, liver, lung and GI tract may also cause elevated CA 15-3 levels.
To make an accurate diagnosis, cancer experts evaluate CA 15-3 test results in conjunction with additional blood tests, physical examinations and diagnostic imaging.
Patients with metastatic breast cancer may have multiple CA 15-3 tumor marker tests during the course of treatment. This is because antigen levels change in response to disease progression or regression. Increasing CA 15-3 levels may suggest the cancer has grown or recurred, while decreasing levels may indicate the cancer is getting smaller.
Fluctuations in CA 15-3 levels are possible in breast cancer patients. As mentioned above, these changes often occur in response to cancer treatment and tumor progression or regression. CA 15-3 levels that remain close to normal levels may suggest the tumor is stable. The patient’s cancer team will discuss test results and how they impact the breast cancer treatment plan and disease prognosis.