The Cancer Antigen 125 (CA-125) test measures the amount of CA-125 in the blood. CA-125 is a protein that is present on the surface of most, but not all, ovarian cancer cells. This makes the test useful as a tumor marker in specific circumstances. Significantly elevated concentrations of CA-125 may be present in the blood of a woman who has ovarian cancer. Small quantities of CA-125 are produced by normal tissues throughout the body and by some other cancers. Levels in the blood may be moderately elevated with a variety of non-cancerous conditions, including menstruation, pregnancy, and pelvic inflammatory disease.