Tumor markers are substances that can often be detected in higher than normal amounts in the blood, urine, or body tissues of some patients with certain types of cancer. These substances can be proteins, enzymes, biochemicals, or antigens. Tumor markers may either be produced by the cancer itself or by the body in response to the cancer. In general, tumor marker levels are lower in early stage disease (but still higher than normal) and higher with advanced disease. Furthermore, their levels decrease in response to treatment and increase when the cancer progresses.
Tumor markers are often used to:
Tumor markers are not specific enough to be used alone for diagnosing cancer. There are several reasons for this:
For these reasons, tumor markers are not used in isolation; instead, results from tumor marker tests are evaluated in the context of a patient’s history, symptoms, and other test results.
Despite these limitations, researchers continue to study the markers in table 1, as well as potential new markers to determine whether they may have a role in screening, early detection, and directing treatment.
Table 1: Tumor markers by cancer type
|Tumor marker||Primary use||Other conditions that cause elevated levels||Normal value
Monitor response to treatment
|CA 19-9||Monitor response to treatment
 Sozzi G, Conte D, Leon M, et al. Quantification of free circulating DNA as a diagnostic marker in lung cancer. Journal of Clinical Oncology 2003;21:3902-3908.