Tests for tumor-specific antigen help diagnose cancer or monitor the effects of treatment in people who have been diagnosed with cancer, explains the National Cancer Institute. As of February 2015, there are more than 20 different tumor markers.
Cancer cells produce substances called tumor markers, states the National Cancer Institute. These markers are found in the blood, waste products and body fluids. Each tumor marker is associated with specific types of tumors. Cancer antigen 19-9 levels may be elevated in cases of gallbladder cancer, stomach cancer or pancreatic cancer. One of the tests used to diagnose ovarian cancer is cancer antigen 125. Some benign conditions cause elevated tumor marker levels, so a high level of a certain cancer antigen is not an absolute indicator of cancer.