Screening tests serve to detect the possibility that a cancer is present before symptoms occur. Screening tests usually are not definitive; results are confirmed or disproved with further examinations and tests. Diagnostic tests are performed once a doctor suspects that a person has cancer.
Tumor markers are substances secreted into the bloodstream by certain tumors. It was first thought that measuring levels of these markers would be an excellent way to screen asymptomatic people for cancer. However, tumor markers are often present to some extent in the blood of people who do not have cancer. Finding a tumor marker does not necessarily mean a person has cancer, and tumor markers have a very limited role in cancer screening.
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