The short answer
Tumor markers are substances, often measured in blood, that can be higher in people with certain cancers. They can help guide treatment decisions and monitor how a cancer responds, but they are not usually enough to diagnose cancer on their own.
Tumor markers are substances, often in blood, that can be higher with certain cancers.
They can help guide treatment and monitor how a cancer responds.
Markers are usually not enough to diagnose cancer by themselves.
Some non-cancer conditions can also raise tumor marker levels.
Watch: Tumor markers, explained
2 min 34 sec · Captioned · PSA, CA-125, CEA — what tumor marker blood tests can and cannot tell you.
Educational only — this video explains general report language and is not medical advice. Only your care team can say what a result means for you.
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The full explanation.
The simple version
A tumor marker is a substance in your blood, urine, or tissue that can give clues about a cancer. Cancer cells make some of these substances themselves. Other tumor markers are made by your body, in response to the cancer. Doctors use them to help diagnose cancer, guide treatment, and check whether treatment is working.
What a tumor marker can tell your doctor
A tumor marker test alone rarely diagnoses cancer. It is one piece of a larger picture, alongside imaging and biopsy results. Once a cancer is diagnosed, a tumor marker can show whether a treatment is shrinking the cancer, since marker levels often drop when treatment works. A rising marker after treatment can be an early sign the cancer is coming back, sometimes before it shows up on a scan.
Common tumor markers and what they are used for
PSA (prostate-specific antigen) is used for prostate cancer. It helps with diagnosis, and doctors track it during and after treatment to watch for recurrence.
CA-125 is used mainly for ovarian cancer. It helps assess whether treatment is working and whether the cancer has returned.
CEA (carcinoembryonic antigen) is used most often for colorectal cancer. Doctors use it mainly to monitor treatment response and watch for recurrence, not to diagnose the cancer in the first place.
AFP (alpha-fetoprotein) is used for liver cancer and certain ovarian and testicular germ cell tumors. It can help with diagnosis, staging, and monitoring treatment.
CA 19-9 is used for pancreatic, gallbladder, bile duct, and stomach cancers, mainly to see whether treatment is working.
HER2 is different from the others. It is not measured in blood; it is tested on tumor tissue. A HER2-positive result means specific targeted drugs, aimed exactly at HER2, are likely to help. This is tested in breast cancer and some other cancers.
How the test is actually done
Most tumor markers are checked with a simple blood draw, the same as routine bloodwork. Some, like HER2, need a piece of tumor tissue instead, usually from a biopsy you already had. Your doctor will tell you which kind applies to your situation, and when in your treatment timeline it gets checked.
Tracking a marker over time
For many cancers, one tumor marker result on its own means less than a pattern over time. Doctors often check a marker before treatment starts, to get a baseline. Then they recheck it during and after treatment. A steady drop is reassuring. A marker that stays flat or climbs again after falling can prompt more imaging, even without symptoms. This is one reason not to skip a scheduled marker test, even when you are feeling well.
Why tumor markers are not perfect
No tumor marker is used alone to diagnose cancer. Some healthy people have naturally higher levels of a marker with no cancer at all. Some people with cancer never show an elevated marker. Levels can also rise from conditions that have nothing to do with cancer, like infection or inflammation. This is exactly why doctors combine marker results with imaging, biopsy, and your symptoms, instead of relying on one number alone.
New tumor markers keep emerging
This is a fast-changing area. New markers, including genetic and molecular tests done directly on tumor tissue, are becoming part of routine care for more cancers. Ask your doctor whether newer marker or biomarker testing is relevant to your specific cancer, since this list keeps growing well beyond the markers used most often today.
Not every cancer has a marker
Many cancers do not have a reliable tumor marker at all. This does not mean your team is missing something. It means imaging, physical exams, and your reported symptoms carry more of the weight in your monitoring plan instead. If your cancer has no standard marker, ask what your team is using to track your progress, so you understand the plan even without a single number to watch.
What to ask your doctor
Ask which tumor marker, if any, applies to your cancer, and what a normal range looks like for you personally. Ask what a rising or falling number would mean for your treatment plan. Ask how often the marker will be checked, and what other tests will be used alongside it.
Sources
Words to know
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Common questions
What is a tumor marker?
A tumor marker is a substance — often a protein — that can be found in higher amounts in the blood, urine, or tissue of some people with cancer. Examples include PSA for prostate and CA-125 for ovarian cancer.
Can a tumor marker diagnose cancer?
Usually not on its own. Markers can rise for reasons other than cancer, and some cancers do not raise them. They are used along with other tests, not as a stand-alone diagnosis.
How are tumor markers used?
They can help guide treatment choices, check how well treatment is working, and watch for recurrence over time by tracking whether the level goes up or down.
What are biomarker tests?
Newer biomarker tests examine the cancer's genes or proteins to help match a person to targeted therapies or immunotherapy. These are increasingly part of planning treatment.
Questions to ask your doctor
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Last updated: 2026-08-05Next planned review: 2028-07-07
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How this page was created
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. We do not employ clinicians and do not intend to — our work 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 checked line by line against the sources listed on it. That confirms the sources support what the page says. 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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