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by Dr. C.H. Weaver M.D. updated 10/2022

What is a CA 125?

CA 125 is a “tumor associated protein” “bio-marker” or “tumor marker” that can be measured in the blood. It can provide useful information about the biological state of ovarian cancer. It is more accurately considered a “tumor associated protein than a “tumor marker” because elevated CA 125 levels do not always indicate the presence of cancer.

How is CA-125 measured?

CA 125 is measured in the laboratory from a simple blood sample. The reference range of CA 125 is 0-35 units/mL (0-35 kU/L). The cutoff of 35 kU/L for CA 125 was determined from the distribution of values in healthy individuals to include 99% of the normal population. If the CA 125 level is higher than normal, it may be due to a benign condition, or the test result could indicate the presence of ovarian, endometrial, peritoneal or fallopian tube cancer.

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What does it mean if CA 125 is elevated above the normal range?

An elevated CA 125 may mean cancer is present however there are also a number of benign conditions that cause elevations of the CA 125 level. Pregnancy, endometriosis, uterine fibroids, pancreatitis, normal menstruation, pelvic inflammatory disease, and cirrhosis of the liver as well as benign tumors or cysts of the ovaries may cause an abnormal test result.

Does a high CA 125 always mean cancer?

No - CA 125 can be absent when cancer is present, and levels can be high when no disease or no malignant disease exists.

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How is CA 125 used in the management of ovarian cancer?

The CA 125 test is used in 4 main ways;

  • Screening: Ca 125 can be used to screen for ovarian, primary peritoneal and fallopian tube cancers in high-risk women, or in women with abnormal findings on examination or ultrasound but not in the population of normal women.CA 125 is not considered to be a screening test for the early detection of ovarian cancer.
  • Monitoring: CA 125 can be used throughout the course of ovarian cancer treatment to monitor the effectiveness of treatment.
  • Prognosis: CA 125 can predict treatment outcomes for some women with ovarian and closely related cancers, such as fallopian tube and primary peritoneal cancer. Some research suggests that low CA 125 levels at diagnosis are associated with improved survival.2
  • Detection: CA 125 is used to detect recurrent ovarian cancer in women who have been previously treated - levels may begin to rise about 90 days before MRI/CT recurrence can be detected.3

NGS Testing will in all likelihood replace CA 125 monitoring because genomic testing can identify treatment options and be followed for cancer recurrence with a liquid biopsy. 

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Does early treatment of a CA 125 recurrence improve outcomes?

The MRC OV05/EORTC 55955 clinical trial evaluated Women in remission after first-line treatment for ovarian cancer. When CA 125 was > 2x the upper limit of normal women either started chemotherapy treatment or waited until there was an objective reason to begin treatment and directly compared. Early treatment of a rising CA125 was associated with a more rapid decline in quality of life and no improvement in survival when compared to delayed treatment.

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  1. Riedinger JM, Wafflart J, Ricolleau G et al. CA 125 Half-life and CA 125 Nadir During Induction Chemotherapy are Independent Predictors of Epithelial Ovarian Cancer Outcome: Results of a French Multicentric Study. Annals of Oncology. 2006;17:1234-1238.
  2. Paramasivam S, Tripcony L, Crandon A et al. Prognostic Importance of Preoperative CA-125 in International Federation of Gynecology and Obstetrics Stage I Epithelial Ovarian Cancer: an Australian Multicenter Study. Journal of Clinical Oncology. 2005;23:5938-5942.