Definition of CA 125

CA 125: A cancer marker, a protein normally made by certain cells in the body including those of the uterine tubes, uterus, cervix, and the lining of the chest and abdominal cavities (the peritoneum and pleura). CA 125 stands for cancer antigen 125.

CA 125 is measured in a blood sample or fluid from the chest or abdominal cavity. All tests in current use are based upon the use of an antibody directed against CA 125 (monoclonal antibody technique).

The normal value for CA 125 is less than 35 kU /ml (in most labs). Without additional information, it is impossible to interpret a high CA 125 since it is increased in so many conditions.

In someone being evaluated for a pelvic mass, a CA 125 level greater than 65 is associated with malignancy is approximately 90% of cases. Without a demonstrable pelvic mass, the association is much weaker.

Increases in CA 125 can also occur with malignancies of the uterine (Fallopian) tubes, endometrium (lining of the uterus), lung, breast, and gastrointestinal tract.

With a known malignancy (such as an ovarian carcinoma) the CA 125 level may be monitored periodically. A decreasing level indicates effective therapy while an increasing level indicates tumor recurrence. Because of test variation, small changes are usually not considered significant. A doubling or halving of the previous value would be important.

Benign conditions that can raise CA 125 include infections of the lining of the abdomen and chest (peritonitis and pleuritis), menstruation, pregnancy, endometriosis, and liver disease. Benign tumors of the ovaries can also cause an abnormal test result.