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Cancer Antigen 125 (CA-125)

Test Overview

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 tumour 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 to see if treatment for cancer is working. If the level of CA-125 is going down, it usually means that the treatment is working.
  • Check to see if ovarian cancer has returned.
  • Check to see if the ovary is the main site of cancer in a woman. If a doctor has found a cancer that has spread to another part of the body (metastatic cancer), he or she may do a CA-125 test to find out where the cancer started. High levels of CA-125 are a strong sign that the cancer started in the ovary. But other types of cancer can increase CA-125 levels too.

The CA-125 test is not recommended as a screening test for ovarian cancer at this time because it often has false-positive results. But the CA-125 test and an ultrasound scan may be used to test women who have a high chance of developing ovarian cancer. Women have a high risk of getting ovarian cancer if they have a family history of ovarian cancer or certain changes (mutations) in their DNA (BRCA1 or BRCA2).

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.


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.


The cancer antigen 125 (CA-125) test measures the amount of the CA-125 protein in the blood.

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Cancer antigen 125 normal values footnote 1



Less than 35 units per millilitre (U/mL)


Less than 35 kiloUnits per litre (kU/L) (SI units)

High values

  • Types of cancer that can cause high CA-125 values include:

What Affects the Test

Results of a cancer antigen 125 test can be affected by:

  • Medicines used to treat cancer.
  • A recent radioactive scan, such as a bone scan.
  • Abdominal surgery. The CA-125 test should be done at least 3 weeks after surgery.

What To Think About

  • The cancer antigen 125 (CA-125) test cannot tell the difference between a cancerous (malignant) or non-cancerous (benign) ovarian lump.
  • The test for CA-125 may also be done on body fluids (other than blood) taken from the chest or stomach area.
  • High values of CA-125 in a woman who has been treated for ovarian cancer may mean that the cancer has returned. Often the high CA-125 level is found many months before the return of cancer can be found in another way.
  • Studies are being done to see if CA-125 can be used as a screening test for ovarian cancer. So far, there is not enough proof to show that this test works to find ovarian cancer early.



  1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.


Current as of: April 29, 2020

Author: Healthwise Staff
Medical Review:
Kathleen Romito MD - Family Medicine
Thomas M. Bailey MD - Family Medicine
Jimmy Ruiz MD - Hematology, Oncology