Summary
What is CA15-3 (cancer antigen 15-3)?
CA15-3 is a test that is used to monitor breast cancer. It is what is called a tumour marker test. These look for substances made by cancer cells or released by the body in response to cancer. By measuring levels of these substances in the blood, tumour marker tests can be used to help monitor cancer treatment.
CA15-3 is a fragment of the protein called MUC1. This is a normal protein that your body makes every day. It belongs to a group of proteins that help protect and lubricate body surfaces. It is mainly made by breast cells. Other types of cells also make small amounts of MUC1, including cells in the ovaries and lungs.
When these tissues are inflamed, stretched, damaged or are rapidly growing, they make more MUC1. This means that CA15-3 is the marker of irritation or activity of tissues that normally produce the protein MUC1.
In breast cancer, breast cells can make and shed large amounts of CA15-3 into the bloodstream, especially as the cancer becomes more advanced.
However, CA15-3 cannot be used to detect or diagnose breast cancer because many non-cancerous conditions as well as several other cancers, cause CA15-3 levels to increase.
Instead, by measuring whether levels of CA15-3 rise or fall, it can be used to see if treatment for breast cancer is working or not and check to see if a cancer is coming back.
CA15-3 rises more in some breast cancers than others mainly because not all breast cancers make and shed the same amount of the MUC1 protein.
Higher CA15-3 rises are typical in hormone-receptor–positive (ER+/PR+) cancers. CA15-3 levels are usually lower in triple-negative breast cancer.
Why get tested?
The CA15-3 test is used to monitor treatment for breast cancer and to make sure the cancer has not come back after treatment. It cannot be used to diagnose breast cancer because other conditions also cause higher levels of CA15-3 in the blood. These include bowel cancer, lung cancer, ovarian cancer, pancreatic cancer, liver disease(cirrhosis), hepatitis and benign breast conditions.
The test is best used to track changes in CA15-3 levels. This can show if your treatment for breast cancer is working.
If you are diagnosed with breast cancer your doctor will measure a baseline CA15-3 level to compare against future measurements. After your surgery and during treatment, CA15-3 testing will be used at intervals to monitor your response. CA15-3 will also be measured from time to time after your treatment is finished to catch any early signs of the cancer’s return. Your results will be considered along with the results of imaging and other blood tests.
CA15-3 is not always measured when breast cancer is detected unless the cancer is already advanced. This is because the majority of early-stage breast cancers do not cause increased levels of CA15-3.
Levels of CA15-3 are not usually taken immediately after breast cancer treatment begins because there have been instances of temporary increases in CA15-3 that do not match the patient’s progress. Your doctor may wait a few weeks after starting treatment to begin monitoring CA15-3 levels.
Diagnosing breast cancer
When breast cancer is suspected, different cancer markers can be used. They cannot make a diagnosis on their own. They include:
If you are diagnosed with breast cancer and have a strong family history of this cancer you may be offered genetic testing for BRCA1 and BRCA2.
Having the test
Sample
Blood.
Any preparation?
None.
Your results
CA15-3 is raised in about 30 percent of people with localised breast cancer and in about 75 percent of those with breast cancer that has metastasised (spread to other organs).
In people whose breast cancer produces CA15 3, higher levels generally reflect a larger amount of tumour, and the levels usually rise as the cancer grows.
In metastatic breast cancer (cancer that has spread to other organs), the highest levels of CA15-3 are often seen when the cancer has spread to the bones and/or the liver.
The CA15-3 level alone is not enough to confirm if treatment is working. Your CA15-3 levels will be considered along with the results of physical examinations, and imaging tests to check how the cancer is responding to treatment.
| CA15-3 level | Possible reasons |
| Normal | Cancer is less likely but not excluded. A normal level of CA15-3 does not rule out breast cancer. Not all breast cancers make it in large amounts. The test is less accurate in early cases when there is less CA15-3 to detect. About 25 percent to 30 percent of people with advanced breast cancer have tumours that do not produce CA15-3. |
| Mild to moderate increases | This could be due to a breast cancer, but mild to moderate increases of CA15-3 are also are seen in a range of other conditions, including liver and pancreatic cancer, cirrhosis and benign breast disorders as well as in some apparently healthy people. The CA15-3 elevations seen in these non-cancerous conditions tend to be stable over time. |
| Moderate–high | Increasing concern for malignancy. |
| Very high | Possibly advanced disease. |
| Falling | A good response to treatment. |
| Rising | Possible progression or recurrence. |
Questions to ask your doctor
The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.
You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.
Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:
More information
Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.
Get further trustworthy health information and advice from healthdirect.
What is Pathology Tests Explained?
Pathology Tests Explained (PTEx) is a not-for profit group managed by a consortium of Australasian medical and scientific organisations.
With up-to-date, evidence-based information about pathology tests it is a leading trusted source for consumers.
Information is prepared and reviewed by practising pathologists and scientists and is entirely free of any commercial influence.
