The short answer
CA 15-3 and CA 27.29 are tumor markers measured in blood. They are used mainly to check how treatment is working in breast cancer, especially advanced breast cancer, or whether the cancer has come back. They are not used to screen for breast cancer, and non-cancer conditions can raise tumor markers.
CA 15-3 and CA 27.29 are tumor markers measured with a blood test.
They are used mainly to follow breast cancer, especially advanced breast cancer.
They are not used to screen for cancer.
Noncancerous conditions can raise tumor markers, and the trend matters more than one value.
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The short version
CA 15-3 is a tumor marker that NCI lists for breast cancer. A related marker, CA 27.29, is listed with it for the same uses. These tests are used to follow breast cancer. They are not used to screen for it.
What CA 15-3 and CA 27.29 are used for
The National Cancer Institute lists CA 15-3 and CA 27.29 as markers used to check whether breast cancer treatment is working or whether the cancer has come back. MedlinePlus notes that they are used to monitor treatment in people with advanced breast cancer.
Why it is not read alone
The National Cancer Institute notes two limits that apply to tumor markers in general. Noncancerous conditions can sometimes raise a marker. And not everyone with a given cancer has a higher level of its marker. So one number is easy to misread. Care teams read it together with imaging, exams, and other results. When a marker is checked again and again during treatment, the pattern over time can help show whether treatment is working.
The National Cancer Institute also notes that circulating tumor markers do not work well for screening. They can miss cancer, and they can suggest cancer in people who don't have it.
Normal ranges vary by lab
Your report lists a reference range, the range that lab calls normal. Labs often use different testing methods, so they often use different ranges. That means results from different labs can't always be compared directly. Your care team knows which lab ran your test and how to read your result.
What to ask
Ask why your care team is measuring CA 15-3 or CA 27.29. Ask what your level and its trend suggest, and whether something other than cancer could explain a change. Their reading of your full picture is what applies to you.
Sources
Common questions
Does a high CA 15-3 mean my cancer is back?
Not by itself. Noncancerous conditions can raise tumor markers, and a single value can be misleading. Your care team reads it with scans, exams, and other results.
What is CA 15-3 used for?
The National Cancer Institute lists CA 15-3 and CA 27.29 as markers used to check whether breast cancer treatment is working or whether the cancer has come back. MedlinePlus notes they are used to monitor treatment in people with advanced breast cancer.
What is the difference between CA 15-3 and CA 27.29?
They are two related breast cancer markers that the National Cancer Institute lists together, for the same uses. Your lab may use one or the other. Ask your care team which one you have.
Can CA 15-3 be used to screen for breast cancer?
No. The National Cancer Institute notes that circulating tumor markers do not work well for screening. They can miss cancer and can be high in people who don't have cancer.
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Sources last checked: 2026-10-02 what this meansLast updated: 2026-09-27Next planned review: 2027-09-27
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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