CA 125 Blood Test

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The CA 125 blood test is a blood test used to measure levels of a specific antigen in the blood. CA 125 is also called cancer antigen 125, and is often elevated when a woman has developed ovarian cancer. The CA 125 blood test is performed by having a phlebotomist or other health care professional draw a sample of blood from a vein in the patient’s arm. This blood sample is then sent to a laboratory, where it is analyzed to determine the concentration of cancer antigen 125.

Test Procedure

This test can also be done on a serial basis; that is, repeated for several consecutive days to allow physicians to determine how fast the concentration of the CA 125 antigen is increasing (Johns Hopkins University). For additional accuracy in diagnosing ovarian cancer using the serialized test, health professionals should be sure to use testing supplies from the same manufacturer and have the test performed using the same laboratory conditions.


Unfortunately, the CA 125 for ovarian cancer blood test is only accurate in detecting about fifty percent of stage I ovarian cancer cases. The use of this test results in many false positive and false negative results. False positives occur when the CA 125 level is elevated, but the patient does not have ovarian cancer. False negatives occur when the CA 125 level is not elevated, and the patient does have ovarian cancer. The consequences of a false positive test can be very serious. Patients may undergo invasive testing or preventive abdominal surgery based on this false test result. The effects of a false negative test can be devastating. Because cancer is not accurately diagnosed, the cancer is given time to become more severe. This reduces the patient’s chances of successful treatment once the cancer is accurately identified (National Cancer Institute).

False Positives

The false positive test result can be caused by a number of health issues including reproductive conditions such as endometriosis and benign reproductive tumors. Inflammation in the pelvic cavity can also cause elevated levels of this cancer antigen (Johns Hopkins University).

Supplemental Testing

For best results, the CA 125 for ovarian cancer blood test should be supplemented with manual pelvic examination and transvaginal ultrasound to help confirm any positive results. In combination with these other diagnostic tools, the CA 125 blood test can be a valuable laboratory tool (National Cancer Institute).


The following reference materials were consulted while preparing this article:

Johns Hopkins University. “Questions and Answers About the CA-125 Test.” Retrieved December 2, 2008. Available: Johns Hopkins University

National Cancer Institute. “Ovarian Cancer Screening Using Ultrasound and CA125 Finds both Early and Late Stage Cancers, But Also Many False Positives.” Retrieved December 2, 2008. Available: National Cancer Institute