Cancer antigen 125 (CA125)

Cancer antigen 125 (CA125) is a protein found on most ovarian cancer cells that is secreted into the blood stream and can be measured. CA125 can also be found on other normal and cancerous cells in the body.

Why a CA125 test is done

A CA125 test may be done:

  • if the doctor suspects there may be cancer
  • to find out if cancer treatment is working
  • to find out if cancer has come back (recurred) after treatment

This test is not recommended as a screening test for ovarian cancer because CA125 can be normal in many women with early stage ovarian cancer and it may be higher than normal in non-cancerous (benign) conditions and other cancers.

How a CA125 test is done

A CA125 test is a blood test that measures the amount of CA125 protein in the blood. A sample of blood is taken by inserting a needle into the vein in your arm. No special preparation is needed.

What the results mean

An increased CA125 value can occur in both cancers and non-cancerous conditions.


The CA125 blood levels can be increased in ovarian cancer and other cancers including:

  • uterine
  • fallopian tube
  • pancreatic
  • breast
  • colorectal
  • lung
  • stomach

A decrease in the CA125 level during treatment usually means that the cancer is responding to treatment.

If the CA125 level stays the same or rises during treatment, it may mean that the cancer is not responding to treatment.

A high CA125 level after treatment is complete may mean that the cancer has come back (recurred).

Non-cancerous conditions

The CA125 blood level may be increased in non-cancerous conditions including:

  • menstruation
  • pregnancy
  • endometriosis
  • pelvic inflammatory disease
  • non-cancerous ovarian cysts
  • uterine fibroids

What happens if the result is abnormal

The doctor will decide whether further tests, procedures, follow-up care or additional treatment are needed.

Expert review and references

  • OneCare Media . CA-125 . Seattle, WA : 2016 :
  • Cannistra SA, Gershenson DM, Recht A . Ovarian cancer, fallopian tube carcinoma, and peritoneal carcinoma. DeVita VT Jr, Lawrence TS, Rosenberg SA. Cancer: Principles and Practice of Oncology. 10th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2015: 76: 1075-1099.
  • Hussain F . Medscape Reference: Gynecologic Tumor Markers Tumor Marker Overview . 2015 :
  • Martin VR, Stewart L . Ovarian cancer. Yarbro CH, Wujcki D, Holmes Gobel B, (eds.). Cancer Nursing: Principles and Practice. 8th ed. Burlington, MA: Jones and Bartlett Learning; 2018: 63: 1793-1827.
  • National Cancer Institute . Tumor Markers . Bethesda, MD : National Cancer Institute ; 2015 :

Medical disclaimer

The information that the Canadian Cancer Society provides does not replace your relationship with your doctor. The information is for your general use, so be sure to talk to a qualified healthcare professional before making medical decisions or if you have questions about your health.

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