CA 15-3 is a protein normally made by breast cells. In clinical practice, it is used as a tumor marker to help assess how advanced breast cancer is responding to treatment.
Blood levels of CA 15-3 are elevated in many people with more advanced breast cancer, especially when the cancer has spread. However, CA 15-3 can also be elevated in some other cancers and in certain noncancerous conditions, so it is not specific enough to diagnose breast cancer on its own.
CA 15-3 results should be interpreted by an oncologist along with other tests, such as molecular testing and tests that evaluate estrogen receptors and HER2. This combined approach helps clinicians make treatment decisions more accurately.
Indications for testing
The CA 15-3 test is indicated to:
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Measure the level of this protein before starting breast cancer treatment.
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Evaluate response to treatment for advanced breast cancer.
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Detect recurrence of breast cancer.
According to the National Cancer Institute (NCI), tumor marker tests are primarily used to monitor treatment response and detect recurrence rather than to screen for cancer. For this reason, CA 15-3 is not used to screen for or diagnose breast cancer.
In addition, CA 15-3 levels may also be increased in other cancers, such as lung, ovarian, or prostate cancer, as well as in noncancerous conditions like benign breast lesions, endometriosis, hepatitis, or pelvic inflammatory disease (PID).
Therefore, CA 15-3 results should always be interpreted by an oncologist alongside other complementary tests.
Test procedure
The test is done using a blood sample only and does not require any special preparation. The sample is collected and sent to a laboratory for processing and analysis.
The analysis process is usually automated and can provide accurate and reliable results quickly.
Reference values
The normal reference range for CA 15-3 is typically 0 to 30 U/mL, and values above 30 U/mL may suggest malignancy. However, reference ranges can vary between laboratories, so results should always be interpreted based on the specific values provided in the lab report.
Interpreting results
CA 15-3 results should be interpreted by an oncologist together with other tests. The test may be repeated several times during breast cancer treatment to help assess how well treatment is working.
In general, the results may indicate:
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CA 15-3 is decreasing: this usually means the breast cancer is responding to treatment and getting smaller.
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CA 15-3 is stable: this may mean the breast cancer is not growing, but it is also not shrinking, and the doctor may decide to change treatment.
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CA 15-3 is increasing: this may mean treatment is not producing the expected results and the breast cancer is progressing, or that the tumor has returned.
In addition, CA 15-3 levels may rise temporarily during the first several weeks after treatment begins, which is why trends over time are more useful than a single result. CA 15-3 is mainly used to monitor treatment response and recurrence in breast cancer.
For this reason, CA 15-3 may be measured multiple times during and after treatment to help monitor effectiveness and support decisions about the best treatment strategy.