What is a Cancer Antigen 125 test?

Cancer antigen 125 (CA) is a protein present on the surface of cancer cells of the ovary; hence, it can be useful as a tumour marker for the detection of ovarian cancer. CA 125 test evaluates the amount of CA 125 antigen in blood.

However, this test lacks specificity and sensitivity, as it is not very accurate in screening ovarian cancer. There are other conditions that can increase CA 125 levels in blood. Also,  certain variants of ovarian cancer cells, which do not exhibit CA 125 antigen. Yet, it is quite useful in detecting ovarian cancer in high-risk patients.

  1. Why is CA 125 test performed?
  2. How do you prepare for CA 125 test?
  3. How is CA 125 test performed?
  4. CA 125 test results and normal range

CA 125 test is done for several reasons:

  • Screening high-risk ovarian cancer patients: CA 125 levels are evaluated along with transvaginal ultrasound every 6 months to yearly and after the age of 40 years to look for the possible signs of ovarian cancer in people who are at high risk for this condition.
  • Monitoring cancer therapy: This test helps check the efficiency of treatment provided for ovarian cancer. Also, it is performed at regular intervals in patients who have a history of fallopian tube, peritoneal or endometrial cancer.
  • Evaluating for recurrence of ovarian cancer: In a previously treated ovarian cancer patient, if CA 125 levels start rising, it indicates a recurrence of the condition.

A CA 125 test alone is never sufficient for detecting or confirming the recurrence of ovarian cancer; it is usually coupled with transvaginal ultrasound, serum epididymis factor 4 level and a computed tomography scan.

Because CA 125 levels may be high in benign or normal physiological conditions, it is not recommended for use as a general screening tool for; but it works well in screening ovarian cancer in high-risk population. Risk factors for ovarian cancer in women include:

  • Obesity
  • Old age (>45 years)
  • Delayed pregnancy (after the age of 35 years) or no pregnancy
  • Infertility treatment
  • Use of hormonal preparations after menopause (hormone replacement therapy)
  • Family history of ovarian cancer, breast cancer or colorectal cancer
  • BRCA1 or BRCA2 gene mutation
  • Smoking
  • Alcohol use
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No special preparations are needed for this test. Inform the doctor about any medications you might be taking, prescription or herbal, alternative, etc. Also, it is important to tell your doctor if you are pregnant or planning to conceive.

It is a simple test that takes less than five minutes. An experienced laboratory specialist collects a blood sample from a vein in your arm by inserting a small needle. A small quantity of blood is then withdrawn into a sterile vial or a test tube. A momentary pricking pain is felt when the needle goes in the vein but the test itself is otherwise painless.

There is also minimal risk of light-headedness and bruising at the site of injection. However, at most times, these symptoms disappear quickly. Rarely, an infection may occur at the site of withdrawal of blood.

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CA 125 levels are measured as units (U)/mL.

Normal results: Normal values of CA 125 in the blood are less than 46 U/nL.

Abnormal results:

Higher values: Higher levels of CA 125 do not necessarily indicate ovarian cancer; values are higher in cases of

If a woman with ovarian cancer (or any cancer of the female genital tract, ie, endometrial cancer or fallopian tube cancer) has decreasing levels of CA 125, it indicates an adequate response to the treatment; however, if the levels of CA 125 are rising despite treatment, it suggests a recurrence or further growth of cancer.

Since, the results of CA 125 test are influenced by several normal conditions (menstruation and pregnancy) or by certain benign conditions (endometriosis, liver disease or even uterine fibroids), it is neither useful for screening general population nor in diagnosing ovarian cancers exclusively.

It is used as an early marker, while other investigations, such as transvaginal ultrasound, pelvic ultrasound, CT scan/ultrasound or CT-guided biopsy are used for the confirmation of ovarian cancer.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor.


  1. Pagana, Kathleen D, Pagana, Timothy J, and Pagana, Theresa N. (©2015) Mosby’s Diagnostic & Laboratory Test Reference 12th Edition: Mosby, Inc. Saint Louis, MO. Pp.199-200.
  2. National Cancer Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Tumor Markers
  3. American Cancer Society [internet]. Atlanta (GA), USA; Ovarian Cancer Risk Factors
  4. Bottoni P, et al. The Role of CA 125 as Tumor Marker: Biochemical and Clinical Aspects.. Advances in Experimental Medicine and Biology. 2015;867:229.PMID: 26530369
  5. MedlinePlus Medical Encyclopedia: US National Library of Medicine; CA-125 blood test

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