CA-125 is a protein that is found more in ovarian cancer cells than in other cells. The protein enters the blood stream and can be easily measured.

This article discusses the blood test done to measure CA-125. The test is used to follow someone during and after ovarian cancer treatment.

A blood sample is needed.

No preparation is necessary.

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing.

The test is often used to monitor women who have been diagnosed with ovarian cancer. The test is useful if the CA-125 level was high when the cancer was first diagnosed. In these cases, the CA-125 test is a very good tool to determine if ovarian cancer treatment is working.

After surgery and chemotherapy, patients should have the test every 2 - 4 months for the first 2 years. This is followed by every 6 months for 3 years, and then yearly.

The CA-125 test may also be done if a woman has symptoms or findings on ultrasound that suggest ovarian cancer.

In general, the CA-125 is not a good test to screen healthy women for ovarian cancer when a diagnosis has not yet been made.

The normal value for a CA-125 depend on the lab running the test. In general, a level above 35 U/mL are considered abnormal.  

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

In a woman with known ovarian cancer, a rise in CA-125 usually means that the disease has progressed or recurred. A decrease in CA-125 usually means the disease is responding to treatment.

In a woman who has not already been diagnosed with ovarian cancer, an elevated CA-125 can mean a number of things. While it can indicate that she has ovarian cancer, it can also indicate other types of cancer, as well as several benign diseases such as endometriosis.

When used in healthy women, an elevated CA-125 usually does not mean ovarian cancer is present. The vast majority of healthy women with an elevated CA-125 do not have ovarian cancer (or any other cancer for that matter).

Any woman with an abnormal CA-125 test will need further tests, and sometimes invasive surgical procedures, to confirm the result. These additional tests all involve risks and anxiety.

Therefore, the CA-125 should not be considered an effective general screening test for ovarian cancer. Studies are underway to determine whether it might be effective when combined with other blood tests or radiologic studies.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

 Coleman RL, Ramirez PT, Gershenson DM. Neoplastic diseases of the ovary: screening, benign and malignant epithelial and germ cell neoplasms, sex-cord stromal tumors. In: Lentz GM, Lobo RA, Gershenson DM, Katz VL, eds.Comprehensive Gynecology. 6th ed. Philadelphia, PA: Elsevier Mosby; 2012:chap 33.