What is a CA-125 blood test?

This test measures the amount of a protein called CA-125 (cancer antigen 125) in a sample of your blood. CA-125 is a type of tumor marker. Tumor markers are substances that are often made by cancer cells or by normal cells in response to cancer.

Organs in the female reproductive system (such as the ovaries and fallopian tubes) may make CA-125 when those organs are inflamed. This may happen in response to normal bodily functions, such as a menstrual period. However, your body may also produce high levels of CA-125 in response to ovarian cancer.

The ovaries are a pair of female reproductive glands that store ova (eggs) and make female hormones. Ovarian cancer happens when the cells in an ovary begin to grow out of control.

Other names: cancer antigen 125, glycoprotein antigen, ovarian cancer antigen, CA-125 tumor marker

What is it used for?

Measuring certain tumor markers often help health care providers treat cancer. So, because high levels of CA-125 can be a sign of ovarian cancer, a CA-125 blood test can help your provider:

• See if ovarian cancer treatment is working or to check if ovarian cancer has come back. This is the most common use of CA-125 blood testing.
• To learn more about a growth or lump in your pelvis (the area below your belly). If a suspicious lump shows up on imaging, such as an ultrasound, your provider may check your CA-125 levels along with other tests to find out whether the lump could be ovarian cancer.
• To screen for ovarian cancer if your risk is very high. If your family health history includes ovarian cancer, your provider may suggest a CA-125 blood test and other tests to look for signs of cancer.

Your provider cannot use a CA-125 to screen for ovarian cancer if you aren't at high risk. That's because other conditions that aren't cancer can also cause high CA-125 levels. These include:

• Endometriosis
• Pelvic inflammatory disease (PID)
• Uterine fibroids
• Liver disease

Natural bodily functions such as your menstrual period and pregnancy can also cause high CA-125 levels.

Why do I need a CA-125 blood test?

If you've been diagnosed with ovarian cancer, you may have several CA-125 blood tests:

• During your treatment to see if your cancer is going away.
• After your treatment, to check if your cancer has returned.

If you have a lump in your pelvis that could be ovarian cancer, your provider may use  a CA-125 test to help find out if it could be ovarian cancer. But only a biopsy can diagnose ovarian cancer.

If you have a very high risk of getting ovarian cancer, your provider may suggest checking your CA-125 levels. If they're high, you'll probably need more tests to find out if you have cancer. You're more likely to get ovarian cancer if you:

• Have a mother or sister, or two or more other relatives who've had ovarian cancer.
• Have family members who've had breast cancer or colorectal cancer (colon cancer).
• Have inherited certain gene changes or conditions that increase your risk of ovarian cancer, such as:
  • Changes in your BRCA 1 or BRCA 2 genes.
  • Lynch syndrome (also called hereditary non-polyposis colorectal cancer), an inherited disorder that increases the risk for many types of cancer.
• Have had breast, uterine (uterus), or colorectal cancer.

If you're concerned about getting ovarian cancer, talk with your provider about your risk.

What happens during a CA-125 blood test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don't need any special preparations for a CA-125 blood test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Your provider will review your CA-125 test results along with other information about your condition. Together, you can discuss how your results affect your diagnosis, treatment, and need for more testing.

In general:

• If you are being treated for ovarian cancer, and the results of several tests show that your CA-125 levels are going down, it usually means that the treatment is helping. If your levels go up or stay the same over time, it may mean that the treatment isn't working.
• If you have finished your treatment for ovarian cancer and your CA-125 levels begin to increase over time, your cancer may be coming back.
• If you have a high risk for ovarian cancer or have a suspicious pelvic lump, high CA-125 levels could be a sign of cancer. Your provider will usually order more tests to make a diagnosis.

While high CA-125 levels don't always mean ovarian cancer, a normal CA-125 test result doesn't rule it out. That's because CA-125 levels may be low in the early stages of cancer. And not everyone with ovarian cancer makes high levels of CA-125.

Talk with your provider if you have questions about your results.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a CA-125 blood test?

The most common type of ovarian cancer is epithelial ovarian cancer. If you have been treated for this type of cancer, you may be tested for a tumor marker called HE4 along with CA-125. Some studies show that measuring both tumor markers provides more accurate information to check whether treatment is working and to look for the return of this type of cancer.