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

This test may be used to monitor certain cancers during and after treatment. In some situations, the test may be used to look for early signs of ovarian cancer in people with a very high risk of the disease.

A CA 125 test isn't accurate enough to use for ovarian cancer screening in general because many conditions can increase the level of this protein.

Conditions that can cause an increase in CA 125 include many that aren't cancerous, such as menstruation and uterine fibroids. Certain cancers may also cause an increased level of CA 125, including ovarian, endometrial, peritoneal and fallopian tube cancers.

Your health care provider may recommend a CA 125 test for several reasons:

• To monitor cancer treatment. If you have ovarian, endometrial, peritoneal or fallopian tube cancer, your provider may recommend a CA 125 test on a regular basis to monitor your condition and treatment.

  But such monitoring hasn't been shown to improve the outcome for those with ovarian cancer, and it might lead to additional and unnecessary rounds of chemotherapy or other treatments.

• To screen for ovarian cancer if you're at high risk. If you have a strong family history of ovarian cancer or you have an inherited gene that increases the risk of ovarian cancer, your provider may recommend a CA 125 test as one way to screen for this cancer.

  Some providers may recommend CA 125 testing combined with transvaginal ultrasound every 6 to 12 months for those at very high risk.

  However, some people with ovarian cancer may not have an increased CA 125 level. And no evidence shows that this screening decreases the chance of dying of ovarian cancer. An elevated level of CA 125 could prompt your provider to put you through unnecessary and possibly harmful tests.

• To check for cancer recurrence. Rising CA 125 levels may indicate that ovarian cancer has come back after treatment. Regular monitoring of CA 125 has not been shown to improve outcomes for those with ovarian cancer and may lead to additional and unnecessary rounds of chemotherapy or other treatments.

If your health care provider suspects that you may have ovarian cancer or another type of cancer, you may undergo additional tests. Other tests that may be helpful in evaluating these cancers include a transvaginal or pelvic ultrasound, serum human epididymis protein 4 (HE4), CT, and MRI. A procedure to remove a sample of cells for testing may be needed to confirm the diagnosis.

If your blood is being tested only for CA 125, you can eat and drink as usual before the test.

For a CA 125 test, a member of your health care team takes a sample of blood by inserting a needle into a vein, usually in the hand or arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.

The results of your CA 125 test will be provided to your health care provider, who will discuss the results with you. Ask your provider when you can expect to know your results.

If your CA 125 level is higher than expected, you may have a condition that isn't cancerous, or the test result could mean that you have ovarian, endometrial, peritoneal or fallopian tube cancer. Your provider may recommend other tests and procedures to determine your diagnosis.

If you've been diagnosed with ovarian, endometrial, peritoneal or fallopian tube cancer, a decreasing CA 125 level often indicates that the cancer is responding to treatment. A rising level may indicate a return or continued growth of the cancer.

A number of conditions that aren't cancerous can cause an elevated CA 125 level, including:

• Endometriosis
• Liver disease
• Menstruation
• Pelvic inflammatory disease
• Pregnancy
• Uterine fibroids