CA 125

Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

• What is CA 125?
• How is CA 125 measured?
• What is the normal range for CA 125?
• What does an elevated CA 125 mean and how is the test used?
• What conditions other than ovarian cancer can cause an increased CA 125?
• Is CA 125 testing useful as a cancer screening test?

What is CA 125?

CA 125 is a protein that is a so-called tumor marker or biomarker, which is a substance that is found in greater concentration in tumor cells than in other cells of the body. In particular, CA 125 is present in greater concentration in ovarian cancer cells than in other cells. Its function is not currently understood. CA stands for cancer antigen.

How is CA 125 measured?

CA 125 is usually measured from a blood sample. It can also be measured in fluid from the chest or abdominal cavity. The tests currently in use are all based upon the use of an antibody that is directed against the CA 125 protein (monoclonal antibody technique).

In 1996, an improved version of the test was introduced and is sometimes denoted as CA 125 - II. The numerical figure of the second generation test results may be higher or lower than a first generation test. When comparing multiple test results over time, it can be important to know which method was used.

What is the normal range for CA 125?

The normal values for CA 125 may vary slightly among individual laboratories. In most laboratories, the normal value is less than 35 U /ml.

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