This is one of the  carbohydrate (glycoprotein) cancer antigens (CA) which may circulate in the blood of patients with ovarian and other adenocarcinomas...casually considered to be an "ovarian  cancer marker"...a serum cancer marker.  An ovarian cancer (OC125) cell line is cultivated and maintained to produce cancer cells which are injected into mice to produce antibodies against the OC125 cells. The murine (mouse) lymphocytes are harvested and grown to produce the antibodies at commercial volumes¹. CA125 increases in serum as a result of any benign or malignant  process which stimulates peritoneal synthesis². It can be helpful in the initial evaluation of a newly discovered mass of the ovary. Were it to be used as a screening test for ovarian cancer, the false positive results, ovarian cancer being fairly rare, would probably outnumber true positive results by over 100 fold. That is, for every 100 women who would be thrown into a panic by an elevated result, no more than 1 would actually have ovarian cancer. Women who want to take charge of their health status can best help themselves by first mapping out a family history of which ancestors and aunts and uncles had what types of cancer. Then if cancer "runs" in the family, particularly breast and ovarian, periodic CA125 screening² and ultrasound examination of the ovaries might be worthwhile.

Conditions Causing Elevations of this Marker:

• 1% of perfectly normal [warning] women¹
• Any benign irritation or inflammation of the abdominal (peritoneal) lining membrane, such as:
  • menstuation²
  • during the several days following monthly rupture of the ovum from the ovary, if there is any hemorrhage
  • rupture of the monthly corpus luteum of the ovary (common)
  • pregnancy²
  • endometriosis (common)^{2, 3}
  • peritoneal effusion for any reason²
  • chronic liver disease, cirrhosis^{2, 3}
  • renal failure and/or UTI^{2, 3}
  • PID (pelvic inflammatory disease...common) or abscess³
  • postoperative peritoneal adhesions that are "irritated"
  • bowel obstruction³
  • chronic peritonitis
• benign multicystic mesothelioma of peritoneum (BMMP)
• other benign tumors lining or protruding above the peritoneum and periodically being mechanically irritated...leiomyoma of uterus^{2, 3}
• non-mucinous tumors of the ovary (benign or malignant)
• primary benign or malignant proliferations or cancers of the peritoneal membranes
• intrauterine endometrial cancer may elevate and >65 U/mL means endometrial cancer has likely gone extrauterine²
• metastatic tumors (such as endometrial, colonic, or breast to the ovaries...Krukenburg tumors) to the peritoneal membranes
• non-abdominal disorders:
  1. pleural effusion^{2, 3}
  2. pulmonary embolism
  3. congestive heart failure²
  4. pneumonia³
  5. Dressler's syndrome³
• when there is an abdominal tumor mass, even high levels do not distinguish benign vs. malignant; and the primary malignancies can be one of any organ or site in the abdominal cavity, including non-Hodgkin's lymphoma² 
• in following an ovarian cancer case postoperatively, CA125 is only 45% sensitive for recurrences.
• preoperative vs 5 day postoperative ratios may be helpful in a number of oncological treatment decisions²
• second-line post-chemo prognostic: a response is defined as a fall in elevated CA-125 levels of at least 50%. The CA-125 criteria were 2.6 times more accurate at predicting survival. On multivariate analysis, the CA-125 response remained a significant predictor of survival, whereas the CT/ultrasound response did not⁴.
• but, once a known recurrence has caused an elevation, it may be a useful monitoring test²

Situations Wherein the Level is Not Elevated:

• any of the above may fail to be elevated because the disorder is too early and for reasons unknown when there is plenty of disease.
• is not a good screening test for serous ovarian cancer because it is not elevated at the time of diagnosis in 20% of cases²
• one cause might be patients with heterophile antibodies in their serum against antibody proteins of the animal (e. g., mouse) that generated the reagent test system antibodies...patient antibodies then block the reagent antibodies and yield a false negative test result

References:

1. The Handbook of Clinical Pathology [text], McKenna & Keffer, 2nd Ed., 2000.

2. Interpretation of Diagnostic Tests, Wallach, 2000, 7th Ed.

3. J. Surg. Oncol. 75:264-265, 2000.

4. J. Clin. Oncology (2004?)...Copenhagen, Denmark group.