Tumor markers are substances that can be detected in higher than normal amounts in the blood, urine, or body tissues of some people with certain types of cancer. A tumor marker may be produced by the tumor itself, or by the body in response to a cancer presence. When diagnosing cancer, blood and pieces of tumor tissue are tested. These tests help determine the characteristics of the tumor (aggressiveness, rate of growth, and degree of abnormality). Tests for tumor markers may be used with other tests or x-rays to detect and diagnose some cancers.

Tumor markers may be proteins, antigens, or hormones. Tumor markers tests are not used alone in 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. 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.