Research
What Is a Tumor Marker? Making Sense of a Term You See in Cancer News
News stories sometimes mention a tumor marker or a blood test that 'detects cancer.' Here's what tumor markers actually are, and what they can and can't tell you.
Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.
What people see in the news
You'll sometimes see stories about a "tumor marker," a blood test that flags cancer, or new "multi-cancer detection" tests that promise to catch cancer early from a single blood draw. It's easy to come away thinking a simple test can give a yes-or-no cancer answer. The reality is more nuanced.
What it actually means
According to the National Cancer Institute, a tumor marker is anything present in or produced by cancer cells — or by other cells of the body in response to cancer or certain noncancerous conditions — that provides information about a cancer, such as how aggressive it is, what treatment it may respond to, or whether it is responding to treatment.
Traditionally, tumor markers have been proteins or other substances made in higher amounts by cancer cells than normal cells. They can show up in blood, urine, stool, tumors, or other tissues and fluids. Increasingly, NCI notes, genomic markers — like tumor gene mutations and patterns of gene expression — are also used as tumor markers.
NCI describes several ways tumor markers are used in cancer care, including helping to diagnose cancer, indicating the type or stage of cancer, estimating prognosis, suggesting which treatments may be effective, showing how well treatment is working, and checking whether cancer has returned after treatment.
One point NCI makes clearly: having an elevated level of a tumor marker does not mean someone has cancer. Noncancerous conditions can raise these levels, and not everyone with a particular cancer will have a higher level of its associated marker. That's why tumor marker measurements are usually combined with other tests, like biopsies or imaging.
What to keep in mind
- A tumor marker result is one piece of information, not a diagnosis on its own.
- NCI states that studies have generally found circulating tumor markers do not work well for screening people who don't have symptoms — they often miss people who have cancer, or flag cancer in people who don't.
- Newer "multi-cancer detection" tests are an active area of research. NCI notes that although several are already being marketed, much remains to be learned about how best to use them and about their harms and benefits.
Questions to ask a healthcare team
- What is this tumor marker test measuring, and why is it being done for me?
- Could something other than cancer explain this result?
- How will this result be combined with other tests before any conclusions are drawn?
- If I'm being monitored, how will changes in this marker over time be interpreted?
Understanding what a tumor marker is — and isn't — makes cancer headlines much less alarming to read. Free, plain-language cancer education helps more people separate a single test result from the fuller picture.