CA-125 is a protein which is elevated in most ovarian cancer cells as compared to normal cells. The CA-125 blood test measures this cancer antigen in the blood and is typically used to determine which ovarian cancer patients are responding to treatment, such as chemotherapy.

Relative to screening, the CA-125 blood test alone isn’t a strong predictor of ovarian cancer as the CA-125 may also elevated in other types of cancer. Additionally, elevated CA-125 levels can be attributed to other conditions besides ovarian cancer, such as: endometriosis, benign ovarian cysts, pancreatitis, pregnancy, pelvic inflammatory disease and even normal menstruation. Some women with ovarian cancer never have elevated CA-125 levels at all. The CA-125 test only has a true positive result in about 50% of Stage I ovarian cancer patients. This increases by stage with an 80% chance of a true positive result in Stage II, III and IV ovarian cancer patients. The remaining 20% of ovarian cancer patients do not show any increase in CA-125 concentrations. Therefore, the CA-125 isn’t used as a screening tool for the general population. Instead, the CA-125 is used in combination with reported symptoms and the other detection methods previously mentioned.

The “normal” range of the CA-125 antigen is 0-35. However, once a woman is diagnosed with ovarian cancer, the absolute value is much less important than the amount of increase or decrease in the number. For example, if a woman’s CA-125 drops in half, that is an encouraging sign that the treatment is working. It is also important to note that one woman with ovarian cancer should not compare her CA-125 value to another woman. Some woman are very sensitive to the CA-125 antigen and others are not. Again, the percentage of increase or decrease is a more important predictor of treatment success.