Tumor markers, also called biomarkers, are measurable substances found in the body that may indicate the presence of cancer when present in large amounts. These substances may include proteins and other biochemicals that can be produced by either tumor cells or by the body in response to tumor cells. In some cases, normal cells also can produce tumor markers, and infrequently they can be found in large amounts in people who do not have cancer.

Tumor markers alone are rarely specific enough to diagnose cancer. Physicians had hoped to use tumor markers to help diagnose cancer in its early stages. However, to date only one marker – prostate-specific antigen (PSA) – has fulfilled that role. Instead, most tumor markers are used to determine the stage of cancer, monitor treatment of advanced cases of cancer, or to detect cancer recurrence after treatment of the disease.  

To determine the presence of a tumor marker, a physician obtains a sample of blood, urine or other tissue from a patient. The sample is sent to a laboratory to determine the presence or level of tumor marker. Tumor markers alone are not used to diagnose cancer. The presence of a tumor marker in higher or lower levels of the substance also can indicate a noncancerous condition, such as inflammation.  

Depending on the type of cancer, additional tests such as blood tests, CAT scans and biopsies must be used to definitively diagnose and stage the disease. Tumor marker tests may be just one component in diagnosing and treating cancer in certain individuals.

Tumor marker testing is continually being studied in many areas of research. The use of tumor marker tests is directed by the U.S. Food and Drug Administration (FDA) and guidelines established by organizations such as the American Cancer Society (ACS) and the American Society of Clinical Oncology (ASCO). The reliability and acceptance of tumor marker tests is not uniform among medical professionals. In recent years, research has focused more on identifying changes in DNA that may indicate cancer or may suggest the best methods to treat certain cancers.