How is it used?CA 15-3 is not sensitive or specific enough to be considered useful as a tool for cancer screening. Its main use is as a tumor marker to monitor a patient's response to breast cancer treatment and to watch for breast cancer recurrence. CA 15-3 can only be used as a marker if the cancer is producing elevated amounts of it; however, since only a small percent of women with localized breast cancer have increased CA 15-3, it may still be able to be used later as a marker.

CA 15-3 sometimes also is used to give a doctor additional information about where the cancer may have spread (such as into the bones or the liver) and a general sense of how much cancer may be present.

When is it ordered? CA 15-3 may be ordered along with other tests, such as estrogen and progesterone receptors, Her2/neu, and BRCA-1 and BRCA-2 genetic testing, when advanced breast cancer is first diagnosed to help determine cancer characteristics and treatment options.

If CA 15-3 is initially elevated, then it may be used to monitor treatment and, if repeated on a regular basis, to detect recurrence. CA 15-3 is usually not done when breast cancer is detected early, before it has metastasized, because levels will not be elevated in the majority of early cancers.