High levels of the cancer antigen 15-3 (CA 15-3) can indicate that breast cancer is present or has spread to other parts of the body. Among other things, the CA 15-3 blood test can reflect how cancer is responding to treatment.
Healthy cells in the breast release a protein called CA 15-3. When a person has breast cancer, cancerous cells or tumors in the breast can release higher levels of CA 15-3 into the bloodstream.
This article explains when doctors may use the CA 15-3 test and what the results can show.
The CA 15-3 test is a blood test to check a person’s CA 15-3 levels. CA 15-3 is a protein that breast cells produce.
Doctors may refer to the CA 15-3 protein as a
When there is a cancerous tumor in the breast, levels of CA 15-3 can increase. This happens because cancerous breast tumors shed higher levels of CA 15-3 into the bloodstream than healthy breast cells.
Doctors may also refer to the test as a cancer antigen breast test or a CA breast test.
Learn more about breast cancer.
A person may have a CA 15-3 test if they are currently undergoing treatment for invasive breast cancer or have had this treatment in the past.
The test can help a healthcare professional determine how well treatment is working or check for breast cancer recurrence. A CA 15-3 test can be a useful test to show whether a tumor is spreading or decreasing in response to treatment.
Doctors may also recommend a CA 15-3 test for people who have received a diagnosis of advanced breast cancer.
According to the Association for Laboratory Medicine, CA 15-3 levels are elevated in approximately 75% of people with breast cancer that has spread to other areas of the body.
Are there alternative tests?
Some people may have a
The CA 15-3 test is a blood test that can take place at a doctor’s office or in a hospital. A healthcare professional will use a needle to take a sample of blood from a vein in the arm.
They will then send the blood sample to a laboratory for testing. Depending on the laboratory, a person may have to wait several days for the results.
Does a person need to prepare for the test?
People do not need to prepare for a CA 15-3 test. However, they will need to make their doctor aware of any medications, supplements, or herbal remedies they are taking ahead of time. A doctor may advise that a person stops taking some substances a few days before any lab tests.
For instance, according to the
The daily recommended biotin allowance for most adults, which is
Additionally, a 2022 study notes that inflammation related to noncancerous health conditions may increase CA 15-3 levels and affect the test results.
A normal test result for CA 15-3 levels is usually 30 units or fewer per milliliter (U/ml).
The Association for Laboratory Medicine notes that the levels of CA 15-3 generally increase in line with the advancement of breast cancer. Increased levels of CA 15-3 may, therefore, indicate that the cancer has spread to other areas of the body.
CA 15-3 levels that continue to increase over time may indicate the following:
- treatment is not working effectively
- cancer is continuing to spread
- cancer is recurring
Decreasing levels of CA 15-3 or levels that return to normal may indicate that treatment is working effectively.
However, in some cases, a person with normal CA 15-3 levels may still have breast cancer. In the early stages of breast cancer, CA 15-3 levels may not be high enough for a test to detect. Also, about 25% to 30% of people with advanced breast cancer have tumors that do not release CA 15-3.
Additional tests can help support the findings of a CA 15-3 test and confirm how cancer is affecting the body.
Laboratories can use different methods to test for CA 15-3, which means results can vary between laboratories.
If a person needs multiple CA 15-3 tests over time, it is best if they ask for the tests to take place in the same laboratory using the same method. This ensures that doctors can interpret and compare the results correctly.
It is important to note that not all doctors monitor the levels of CA 15-3, as other tumor markers are more sensitive. A 2018 study found that the tissue polypeptide-specific antigen was the best candidate for predicting the recurrence of breast cancer.
In addition, the CA 15-3 test is not always a reliable indicator. Generally, increasing levels of CA 15-3 indicate that breast cancer is advancing or recurring, while decreasing levels suggest that treatment is working effectively to reduce cancer. However, other factors, such as other medical conditions, can affect levels of CA 15-3.
Research from 2020 that focused on females who did not have breast cancer found that lower levels of CA 15-3 may be present in those who:
- are perimenopausal
- have a history of uterine fibroids
- have taken birth control pills for an extended period
The researchers also found that elevated levels of CA 15-3 were present in Hispanic and Black females, as well as those with a history of endometriosis or lung cancer.
Doctors will use other tests to confirm any findings from a CA 15-3 test in order to make an accurate diagnosis or assess how the treatment is working.
There are minimal risks related to the CA 15-3 test. People may experience the following at the needle insertion point:
Some people may feel lightheaded after the test. A healthcare professional will check that a person is feeling well before they leave. However, it is important to contact a doctor if there are any signs of infection or severe symptoms afterward.
According to a 2022 article, false-positive results can occur in people with benign conditions, suggesting that tumor markers may not always be accurate indicator of cancer. A person may experience emotional distress after receiving a false-positive test result.
A CA 15-3 test is a blood test that doctors may use in people with a current or previous breast cancer diagnosis. The CA 15-3 test shows the levels of CA 15-3 protein in the blood, which can increase as breast cancer advances.
Decreasing levels of CA 15-3 may indicate that treatment is working effectively to reduce the spread of cancer. However, CA 15-3 can sometimes indicate other health issues, such as liver disease or benign conditions of the breast. For this reason, doctors typically use other tests to confirm the findings.
