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How Do Tumor Markers Work?

When diagnosing cancer, blood and pieces of tumor tissue are tested. These tests help to determine the characteristics of the tumor (aggressiveness, rate of growth, and degree of abnormality). Tests for tumor markers may be used with other laboratory tests or procedures such as x-rays to detect and diagnose some cancers.

Tumor markers may be proteins, antigens, or hormones. Tumor marker tests are not used by themselves for making a cancer diagnosis because most markers can be found in elevated levels in people who have benign conditions, and because no tumor marker is specific to a particular cancer.

How tumor markers are measured

Tumor markers are most often measured in blood and urine. They can also be found in tumors and other tissues.

Tumor markers are sent to a laboratory where various methods are used to measure the levels. The marker is usually found by combining the sample with man-made antibodies that react with the tumor marker protein.

Are tumor markers reliable indicators of cancer?

Not every tumor will cause an elevation in the tumor marker test, especially in the early stages of cancer. Physicians can use changes in tumor marker levels to follow the course of the disease, to measure the effect of treatment, and to check for recurrence.

Tumor markers are not always reliable for the following reasons:

  • Most tumor markers can be made by normal cells, as well as cancer cells.
  • Tumor markers can be associated with noncancerous conditions.
  • Tumor markers are not always present in early stage cancers.
  • People with cancer may never have elevated tumor markers.
  • Even when tumor marker levels are high, they are not specific enough.

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Reliability also depends on test sensitivity and specificity

Tumor marker sensitivity refers to the test's ability to identify people who have the disease. If a test is not very sensitive, there will be many "false negative" results, and persons with cancer will go undetected.

A test that yields many false negatives will obviously not be very good at helping reduce cancer deaths and creates a false sense of security for people who actually have the disease.


This image shows 10 green spots on top that are actual cancer cases and 10 green spots on the bottom that are also cancer cases. In the bottom image only 7 of the 10 cancers have been found - the other three indicate "false negative" results that were not detected.


Image Courtesy of the National Cancer Institute


Tumor marker specificity refers to the test's ability to identify people who do not have the disease. If a cancer test is not very specific, it will yield many "false positive" results where a person will test positive even though they are cancer free.


This error can lead to unnecessary and costly follow-up procedures and can cause anxiety in the person misdiagnosed.

This image depicts many false positive results.


Image Courtesy of the National Cancer Institute

How tumor markers are used in cancer care

Because abnormal tumor marker levels may only suggest the presence of cancer, other scientific tests are usually required before confirming a cancer diagnosis. Clinicians use tests such as biopsies and the evaluation of multiple tumor markers with some cancer types. A patient's history, physical exam, and other lab tests are also considered.

Tumor markers are measured over a period of time to see if the levels are increasing or decreasing. Serial measurements are often more meaningful than a single measurement. It is also best to compare results from the same lab.

Tumor markers are primarily used to follow people who have already been diagnosed with cancer. In monitoring people with cancer, tumor markers can be less expensive and invasive than other diagnostic tools. Tumor markers can also help doctors figure out where a cancer started when the disease, when found, is already widespread.

Tests measuring tumor markers help in diagnosing cancer, monitoring treatment efficacy, and disease status before, during and after therapies, and evaluating the possibility of recurrence.


It is thought that combining the results from several markers may be better than using a single marker.


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