What Are Tumor Markers? A Plain-Language Guide

This page is for anyone who has heard the term "tumor marker" and wants to know what it means. It covers what these tests show, the common ones, and their limits.

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. Others 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.

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.

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 targeted drugs aimed at HER2 are likely to help. This is tested in breast cancer and some other cancers.

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 you, and when it will be checked.

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 to keep scheduled marker tests, even when you feel well. Ask your care team about your testing schedule.

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 why doctors combine marker results with imaging, biopsy, and your symptoms, instead of relying on one number alone.

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 fits your cancer, since this list keeps growing.

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.

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.

• National Cancer Institute — Tumor Markers.