- 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 .
- 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.
- In general, the higher the CA 15-3 level the more advanced the breast cancer and the larger the tumor burden (amount of tumor present). The level tends to increase as the cancer grows. In metastatic breast cancer, the highest levels of CA 15-3 often are seen when the cancer has spread to the bones and/or the liver.
- Mild to moderate elevations of CA 15-3 also are seen in a variety of conditions, including liver and pancreatic cancer, cirrhosis, and benign breast disorders as well as in a certain percentage of apparently healthy individuals. The CA 15-3 elevations seen in these non-cancerous conditions tend to be stable over time.
- Negative CA 15-3 levels do not ensure that a patient does not have cancer. It may be too soon in the disease for elevated levels to be detected. In addition, 25% to 30% of individuals with advanced breast cancer have tumors that do not shed CA 15-3.
