The cancer antigen 125 (CA-125) test is a blood test. It measures a protein in the bloodstream that can indicate various health conditions. These include certain types of cancer, such as ovarian cancer, and some benign conditions, such as endometriosis.

The CA-125 test measures a certain protein in the blood that can indicate ovarian, endometrial, or fallopian tube cancer. Doctors may also use CA-125 tests to monitor the effectiveness of treatment for these cancers.

High CA-125 levels can also signify other conditions, such as endometriosis or diverticulitis, and pregnancy.

In this article, we look at what the CA-125 test involves, when doctors may use it, and what the results may mean.

The CA-125 test is a blood test that doctors may use to diagnose or monitor certain types of cancer, including ovarian cancer and endometrial cancer.

Cells produce a protein called CA-125 on their surface, which then releases into the bloodstream. CA-125 is a tumor marker, or biomarker, which means that elevated levels can indicate the presence of cancer in the body.

More than 80% of people with advanced ovarian cancer and 50% of those with early stage ovarian cancers have elevated CA-125 levels. Noncancerous conditions, such as endometriosis or uterine fibroids, can also cause an elevation in CA-125 levels.

Doctors may use a CA-125 test if they suspect that a person has ovarian cancer. The test result may help them make a diagnosis.

If people are receiving treatment for ovarian cancer, a CA-125 test can help doctors monitor how well their treatment is working. Decreasing CA-125 levels can indicate that treatment is working, whereas CA-125 levels that are staying the same or increasing may indicate the need for an alternative treatment.

Doctors may also use the CA-125 test alongside other tests to determine whether ovarian cancer has recurred.

Learn about the early symptoms of ovarian cancer.

People who have symptoms of ovarian cancer or are receiving treatment for the disease may have a CA-125 test.

According to 2021 research, the CA-125 test is not as helpful in diagnosing early stage ovarian cancer due to low sensitivity. The results are also less specific in people who are premenopausal, so the test is more useful for people who are postmenopausal.

According to the Ovarian Cancer Research Alliance (OCRA), the CA-125 test is most accurate for testing people who are postmenopausal and have a pelvic mass.

The Canadian Cancer Society states that the CA-125 test is not a recommended test for ovarian cancer screening. This is because CA-125 levels can be normal in people with early stage ovarian cancer. Also, benign causes can be responsible for higher levels in some cases.

The United States Preventive Services Task Force agrees that the negatives of screening, including the risk of false positives, outweigh the benefits, concluding that “screening for ovarian cancer does not reduce ovarian cancer mortality.”

The CA-125 test is a blood test, so a doctor will insert a needle into a vein in the arm to take a blood sample. A doctor will then send the sample to a laboratory where pathologists will analyze the sample.

Once a doctor receives the results, they will contact the person to explain what the results mean and outline any next steps.

No preparation is necessary for a CA-125 test, and people will be able to eat and drink as usual beforehand. They will also be able to resume their normal activities afterward.

Menstruation can cause CA-125 levels to rise, so it is best to avoid having a CA-125 test while menstruating. CA-125 levels are typically higher when there is inflammation in the pelvic region. Therefore, other noncancerous conditions and factors that can cause CA-125 levels to rise include:

Cancers other than ovarian cancer that may cause a rise in CA-125 levels include:

According to OCRA, a normal level of CA-125 is 0–35 units per milliliter (U/ml).

Doctors consider any level above 35 U/ml to be a high CA-125 level.

High CA-125 levels are commonly a sign of a noncancerous condition.

However, high CA-125 levels may sometimes indicate a type of cancer, such as ovarian, endometrial, or fallopian tube cancer. It is also possible for people to have ovarian cancer without having abnormally high CA-125 levels.

In people with endometrial cancer, very high CA-125 levels may indicate that the cancer has spread outside the uterus. Doctors can also use CA-125 results to check how well surgery has worked to remove the cancer.

If people are receiving treatment for cancer, decreasing CA-125 levels may mean that the treatment is working. Doctors will determine this by comparing the new levels against the original ones.

A CA-125 test is a blood test that measures the amount of CA-125 protein in the bloodstream.

High levels of CA-125 can signify certain health conditions, including uterine fibroids, endometriosis, and some cancers, such as ovarian cancer. Menstruation and pregnancy can also result in elevated CA-125 levels.

Doctors may use a CA-125 test if they suspect that a person has ovarian cancer. They may also use the test to monitor the effectiveness of treatment for ovarian or endometrial cancer.

If people have abnormal results from a CA-125 test, a doctor may conduct further tests to check for the underlying cause and suggest any necessary treatment.