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Blood Test Tracker

CA-125 (Cancer Antigen 125)

A protein made by tissue lining the pelvis and belly. Doctors mainly use it to follow ovarian cancer, and many harmless conditions raise it too.

AI-verified against cited sources. Awaiting expert reviewAlso called CA-125, CA 125, CA125LOINC 10334-1

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The usual range for CA-125 depends on sex. Choose female or male above to see where you stand.

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What it is

CA-125 is a protein found on cells that line the ovaries, the fallopian tubes and the inside of the belly and chest. A small amount is normally in the blood. The amount can rise when these tissues are irritated, inflamed or growing. Common examples are a menstrual period, endometriosis and ovarian cancer. Labcorp says no range is established for men, and Mayo says the test does not apply to them.

Why it matters

The most common use is to follow people who have ovarian cancer, to see if treatment is working or if the cancer has come back. It is also used with an ultrasound or other imaging to learn more about a lump found in the pelvis, and sometimes in people at very high inherited risk. It isn't a good test for people at average risk, because many harmless conditions raise it and many early ovarian cancers don't. The U.S. Preventive Services Task Force recommends against screening for ovarian cancer in women without symptoms who aren't at high risk.

If it’s high

A result above your lab's range does not mean you have cancer. Periods, endometriosis, fibroids, pelvic infections, pregnancy and liver disease can all raise CA-125. Higher values are more common in people with ovarian cancer and some other cancers. Your clinician will look at the number together with your symptoms, an exam and imaging.

Common causes

  • Your menstrual period (levels rise around menstruation)
  • Endometriosis
  • Uterine fibroids
  • Ovarian cysts
  • Pelvic inflammatory disease or inflammation of the cervix
  • Pregnancy, especially the first trimester, and around delivery
  • Liver disease such as cirrhosis or hepatitis
  • Fluid in the belly (ascites) from many different causes
  • Inflammation of the lining around the heart (pericarditis)
  • Inflammation of the pancreas, reduced kidney function, or autoimmune conditions
  • Ovarian cancer, and some other cancers such as uterine, lung, breast, pancreatic or digestive cancers

What can help

  • Ask your clinician whether the timing of the test in your cycle matters if you still have periods
  • Tell your clinician about endometriosis, fibroids, liver disease, pregnancy, and recent surgery or chemotherapy
  • Ask whether an ultrasound or other imaging would help explain the number
  • Use the same lab and method for follow-up tests

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex. Choose women or men to see the one that applies.

What can affect your result

  • Menstrual cycle: levels are higher during menstruation, and higher in women who still have periods than after menopause
  • Pregnancy: levels can rise early in pregnancy and around delivery
  • Age: levels tend to fall as women get older
  • Some studies found lower levels in women of African or Asian ancestry than in white women, in smokers and people who regularly use caffeine, and after a hysterectomy
  • Higher body weight is linked with higher levels in some studies
  • Recent chemotherapy or abdominal surgery
  • Assay method: results from different methods can't be compared directly
  • Biotin supplements and antibodies to mouse proteins can interfere with some methods
  • High-dose biotin (vitamin B7, found in many hair, skin and nail supplements) can interfere with some immunoassays. Mayo asks people to avoid biotin for 12 hours before the draw, and Labcorp asks for at least 72 hours.
  • Antibodies to mouse proteins (HAMA), for example after some scans or treatments that use mouse antibodies, can cause wrong results on some methods.
  • Labcorp suggests waiting at least 3 weeks after finishing chemotherapy and at least 2 months after abdominal surgery before testing.
  • Periods, pregnancy (especially the first trimester and around delivery), endometriosis, fibroids, pelvic infections, liver disease and fluid in the belly can raise CA-125 without cancer.
  • Results from different methods aren't interchangeable, so use the same lab and method when following results over time.
  • Mayo reports no high-dose hook effect up to 50,000 U/mL on its method.

CA-125 has a half-life of about 5 days (Mayo), so changes after treatment show over a few weeks.

When to talk to a clinician

Talk with your clinician soon if your CA-125 is above your lab's range and you have symptoms that are new and don't go away. These include belly or pelvic pain, swelling or pressure, trouble eating or feeling full, a sudden or frequent need to urinate, or ongoing bloating. If you are being followed for ovarian cancer, tell your care team about any rising result. Get urgent care for sudden severe belly or pelvic pain.

Questions you could ask

  • Why was CA-125 ordered for me, and how will we use the result?
  • If I'm being followed for ovarian cancer, how often should it be checked, and should I use the same lab each time?
  • Could my period, endometriosis, fibroids or another non-cancer condition explain this number?
  • Should we repeat it, and does the timing in my cycle matter?
  • Would an ultrasound or other imaging help?

Questions people ask

Can CA-125 be high if I don't have cancer?

Yes, and this is common. Periods, endometriosis, fibroids, pelvic infections, pregnancy and liver disease can all raise it. A result above range is a reason for your clinician to look more closely, not a diagnosis. And a result in range doesn't rule out ovarian cancer.

Should I get a CA-125 test to check for ovarian cancer?

For women without symptoms who aren't at high risk, the U.S. Preventive Services Task Force recommends against screening for ovarian cancer. Screening was not shown to save lives and led to unnecessary surgery in some women. If you have a strong family history, an inherited gene change, or symptoms that don't go away, talk with your clinician about what makes sense for you.

Why does my lab's range differ from 35?

Labs use different methods. Quest prints under 35 U/mL, Labcorp prints 0.0 to 38.1 for women, and Mayo Clinic Laboratories prints under 46. Values from different methods can't be compared directly, so use the range on your own report.

Show the science and sources

Units

  • 1 U/mL = 1 U/mL
  • 1 U/mL = 1 kU/L

Is a change real?

CA-125 naturally varies about 6.6% from test to test in the same healthy person. Between two results from the same lab, a rise of 23% or more, or a fall of 18% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical review and meta-analysis of biological variation estimates for tumor markers (Marques-Garcia F et al., EFLM WG-BV).

For clinicians and researchers

CA-125 is the OC125-defined epitope on MUC16, a high-molecular-weight glycoprotein (200 to 1,000 kDa) expressed by tissues derived from coelomic epithelium. Its half-life is about 5 days, and Mayo notes that a half-life over 20 days during chemotherapy may be linked to shorter disease-free survival. Mayo reports it elevated in about 80% of women with advanced epithelial ovarian cancer, with average sensitivity of about 50% for stage I and 90% for stage II or higher, and a 2020 review notes that about 20% of ovarian cancers don't express it. The 35 U/mL cutoff comes from a 1983 radioimmunoassay study in which about 1% of healthy women were above it. The review also cites about 5% of healthy women above 35 and 0.1% above 100, and a positive predictive value of about 1% for a 35 U/mL cutoff in asymptomatic women. The same review says ACOG recommends a 200 U/mL cutoff for premenopausal women with pelvic masses, on expert opinion, because the 35 U/mL cutoff gives more false positives before menopause. The NACB 2008 guideline recommends CA-125 with transvaginal ultrasound for early detection in women at high risk, for the differential diagnosis of suspicious pelvic masses in postmenopausal women, and for detecting recurrence, monitoring therapy and prognosis in ovarian cancer. The USPSTF (2018) gives a grade D recommendation against screening asymptomatic women who aren't at high risk. Methods: Labcorp and Mayo run the Roche Elecsys CA 125 II electrochemiluminescence immunoassay; Quest lists an immunoassay, and the VITROS assay is a different method. Mayo says values from different methods can't be used interchangeably. The VITROS FDA summary reports no hook effect up to 340,000 U/mL, defines progressive disease as a rise of at least 25% (to be repeated within 2 to 4 weeks) and partial remission as a fall of at least 50%. Reference limits: Quest under 35 U/mL, VITROS 35 U/mL or lower, Labcorp 0.0 to 38.1 U/mL for women, Mayo under 46 U/mL for women; men have no established range. Many non-cancer conditions raise CA-125, including menstruation, endometriosis, fibroids, ovarian cysts, pelvic inflammatory disease, early pregnancy, cirrhosis, hepatitis, pericarditis, pancreatitis, renal insufficiency and autoimmune disease (Labcorp, Mayo, FDA summary).

LOINC 10334-1 (primary), 83082-8. Specimen: serum.

Sources

  1. CA 125 (test code 29256), Quest Diagnostics
  2. VITROS CA 125 II Reagent Pack: FDA 510(k) summary K221355, Ortho Clinical Diagnostics (FDA 510(k) database) (2022)
  3. Cancer Antigen (CA) 125 (test 002303), Labcorp
  4. CA25 - Overview: Cancer Antigen 125 (CA 125), Serum, Mayo Clinic Laboratories
  5. CA-125 Blood Test (Ovarian Cancer), MedlinePlus (National Library of Medicine) (2025)
  6. Ovarian Cancer, MedlinePlus (National Library of Medicine)
  7. Screening for Ovarian Cancer: US Preventive Services Task Force Recommendation Statement, JAMA / US Preventive Services Task Force (2018)
  8. CA125 and Ovarian Cancer: A Comprehensive Review, Cancers (2020)
  9. National Academy of Clinical Biochemistry laboratory medicine practice guidelines for use of tumor markers in testicular, prostate, colorectal, breast, and ovarian cancers, Clinical Chemistry (2008)
  10. Tumor Marker Tests in Common Use, National Cancer Institute (2023)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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