CA 15-3 (Cancer Antigen 15-3)

1. What is the purpose of this test?

The cancer antigen 15-3 (CA 15-3) test, along with the related CA 27.29 test, is primarily employed to monitor a patient's response to breast cancer treatment and to aid in the surveillance of breast cancer recurrence. These tests are utilized in conjunction with other clinical assessments and breast cancer-related tests like estrogen and progesterone receptor assays, Her2/neu status evaluation, and gene expression tests (e.g., Oncotype DX, MammoPrint) to assess the status of a patient's breast cancer.

In some cases, CA 15-3 may be ordered to provide a rough estimate of the extent of cancer present in the body (tumor burden). It's important to note that CA 15-3 can only serve as a marker if the cancer is actively producing elevated levels of this antigen, so it may not be applicable to all breast cancer patients.

The CA 15-3 and CA 27.29 tests are not suitable for use as general cancer screening tests due to their lack of sensitivity and specificity. Elevated levels of these antigens can also be caused by non-cancerous conditions.

2. When is this test prescribed?

CA 15-3 may be initially ordered along with other tests when advanced breast cancer is diagnosed to assess cancer characteristics and guide treatment decisions. Subsequently, if the CA 15-3 levels are initially elevated, the test may be periodically ordered to monitor the treatment's effectiveness and to surveil for potential recurrence. Typically, CA 15-3 is not requested in cases of early breast cancer detection, before metastasis occurs, as elevated levels are uncommon in the majority of early-stage cancers.

3. What does the test result mean?

In general, higher CA 15-3 levels are associated with more advanced breast cancer and a larger tumor burden. As breast cancer progresses, CA 15-3 concentrations typically increase, with the highest levels often observed in cases of metastatic breast cancer that has spread to the bones and/or liver.

A rise in CA 15-3 concentrations over time may signal inadequate response to treatment or cancer recurrence.

However, normal CA 15-3 levels do not guarantee the absence of localized or metastatic breast cancer. Elevated CA 15-3 levels may not yet be detectable in early disease stages, or approximately 20% to 25% of individuals with advanced breast cancer may not shed CA 15-3.

Mild to moderate elevations of CA 15-3 are also observed in various non-cancerous conditions, such as lung, pancreas, ovary, prostate, and colon cancers, cirrhosis, hepatitis, benign breast disorders, and in a subset of apparently healthy individuals. In these non-cancerous conditions, CA 15-3 elevations tend to remain relatively stable over time.

4. I have a strong family history of breast cancer. Shouldn't I be screened for CA 15-3?

CA 15-3 is not recommended for use as a screening tool, as it lacks the necessary specificity and sensitivity to detect early breast cancer. Elevated CA 15-3 levels may be attributed to various non-cancerous conditions, and a normal value does not guarantee the absence of cancer. Typically, CA 15-3 should be employed after breast cancer has already been diagnosed.