CA 125 is a cell surface protein often found on ovarian cancer cells and in some normal tissues. It is one of the protein antigens that has been associated with the presence of various cancers, but is most often positive in ovarian cancer. CA 125 concentrations may also be elevated in other types of non-cancerous conditions, including menstruation, pregnancy, and pelvic inflammatory disease. CA 125 is a useful tumor marker for evaluating therapy and monitoring disease status in ovarian cancer patients. As the levels decrease it is an indication that the therapy or treatment is working. Post-operatively, the CA 125 concentration correlates with tumor bulk and is a prognostic indicator of clinical outcome. It has been reported that patients with levels exceeding 35 U/mL have the highest risk for clinical recurrence. CA 125 is not useful as a screening mechanism because it is elevated in many non-cancerous conditions, although women that are at higher risk due to family history or other reasons are usually tested regularly as a precautionary method.
Measured serially, serum CA 125 concentration typically corresponds with disease progression or regression. The rate of change in CA 125 is also highly prognostic. A rapid decrease in the level of CA 125 indicates a positive response to treatment. Elevated levels of CA 125 after the third course of primary chemotherapy are typically predictive of poor outcomes.