A cancer antigen 125 (CA-125) is a protein found on the surface of many ovarian cancer cells. It also can be found in other cancers and in small amounts in normal tissue. A CA-125 test measures the amount of this protein in the blood.

CA-125 is used as a tumor marker-which means the test can help show if some types of cancer are present. Most often, the CA-125 test is used to check how well treatment for ovarian cancer is working or to see if ovarian cancer has returned.

Why It Is Done

The test for cancer antigen 125 (CA-125) is used to:

• Check if treatment for cancer is working. If the level of CA-125 is going down, it usually means that the treatment is working.
• Check if cancer has returned.
• Check if the ovary is the main site of cancer in a woman. If a cancer is found at another site (metastatic cancer), a higher-than-normal CA-125 test result may mean that ovarian cancer is the main site. However, other types of cancer also can increase CA-125 levels.

The CA-125 test is not recommended as a screening test for ovarian cancer at this time. However, the CA-125 test and an ultrasound scan may be used to test women who have a high chance for ovarian cancer. Women have a high risk of ovarian cancer if they have a family history of ovarian cancer or certain changes (mutations) in their DNA (such as the breast cancer BRCA1 mutation).

How To Prepare

You do not need to do anything before you have this test.

How It Is Done

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then put on a bandage.

How It Feels

The elastic band around your upper arm may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very small chance of problems from having blood drawn from a vein.

• You may get a small bruise at the site. You can lower your chance of bruising by putting pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This condition is called phlebitis. You can use a warm compress several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin ( Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.