What this test measures
CA-125 (Cancer Antigen 125, MUC16) is a large glycoprotein expressed on the surface of ovarian epithelial cancer cells and shed into the bloodstream. The test measures CA-125 in serum by immunoassay.
The marker is elevated in over 80% of patients with epithelial ovarian cancer at presentation, particularly in advanced (stage III–IV) disease. But it is also raised in many benign conditions — endometriosis, fibroids, pelvic inflammatory disease, menstruation, early pregnancy, liver disease, pleural / peritoneal effusion — limiting specificity, especially in pre-menopausal women.
Why it matters
CA-125 has well-defined clinical uses:
• Monitoring response to chemotherapy in confirmed ovarian cancer • Detecting recurrence after curative-intent treatment • Helping assess pelvic masses (especially in post-menopausal women) as part of the Risk of Malignancy Index (RMI) and ROMA score • In suspected ovarian cancer workup, alongside imaging and clinical exam
It is explicitly NOT recommended as a screening test in average-risk asymptomatic women. The PLCO and UKCTOCS trials showed that population screening with CA-125 ± ultrasound does not reduce ovarian cancer mortality and increases false-positive surgeries. For BRCA mutation carriers and women with strong family history, CA-125 may be used selectively as part of surveillance, although prophylactic risk-reducing surgery is the recommended risk management for these high-risk women.
How to prepare
No fasting required. Standard venous blood sample. For pre-menopausal women, ideally schedule the test 1–2 weeks after the start of menstruation — CA-125 rises during menstruation and can give misleadingly high results. Mention any history of endometriosis, fibroids, pregnancy, recent abdominal surgery, peritonitis, pleural effusion, or ascites — these all elevate CA-125 modestly.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| CA-125 (U/mL)[1][2][3] | < 35 U/mL (lab-specific) | Low / normal CA-125 in a treated ovarian cancer patient is reassuring but does not exclude recurrence. Up to 20% of epithelial ovarian cancers do not produce CA-125. | 35–200 U/mL: nonspecific elevation in pre-menopausal women — endometriosis, fibroids, menstruation, pelvic inflammatory disease. 200–500 U/mL in a post-menopausal woman with pelvic mass: concerning for ovarian cancer. > 500 U/mL: strong suspicion of ovarian or other intraperitoneal cancer. |
CA-125 — when appropriate and when not
| Context | Use CA-125? | Notes |
|---|---|---|
| Screening asymptomatic average-risk women | No | UKCTOCS / PLCO show no mortality benefit |
| Post-menopausal pelvic mass | Yes | Part of RMI / ROMA risk stratification |
| Pre-menopausal pelvic mass | Selective | High false-positive rate; interpret with caution |
| Monitoring known ovarian cancer | Yes | Track response to chemotherapy |
| Recurrence surveillance | Yes | Combined with imaging and clinical exam |
| BRCA carrier surveillance | Selective | Often combined with transvaginal ultrasound |
Frequently asked questions
Can CA-125 detect ovarian cancer early?
Not reliably. The large UKCTOCS and PLCO trials showed that screening with CA-125 ± ultrasound does not reduce ovarian cancer mortality in average-risk women — and causes false-positive surgeries. Ovarian cancer screening is not currently recommended for the average-risk asymptomatic population.
My CA-125 is elevated and I have endometriosis — should I worry?
Endometriosis commonly raises CA-125 (often 35–200 U/mL). In a pre-menopausal woman with known endometriosis, mild elevation is usually from the endometriosis itself. Persistent elevation, post-menopausal status, or rising values warrant imaging.
When is CA-125 most useful?
In a post-menopausal woman with a pelvic mass (helps differentiate benign from malignant). In monitoring response to chemotherapy in confirmed ovarian cancer. In recurrence surveillance after curative treatment.
I have a BRCA mutation — should I get CA-125 regularly?
BRCA1/2 carriers may have CA-125 plus transvaginal ultrasound every 6 months as part of surveillance, but the more effective risk management is risk-reducing salpingo-oophorectomy after childbearing. Discuss with a genetic counsellor and gynaecologic oncologist.
Why is CA-125 less specific in pre-menopausal women?
Many common benign pelvic conditions in pre-menopausal women (endometriosis, fibroids, ovulation, menstruation, pelvic inflammatory disease) raise CA-125 — leading to false positives.
When during my menstrual cycle should I test?
Ideally 1–2 weeks after the start of menstruation, when CA-125 is at its lowest baseline.
How long does the report take?
Most NABL labs deliver CA-125 results in 24–48 hours.
Related Oncology / Tumor Markers tests
Tests commonly ordered alongside CA-125, or that help interpret an unexpected result.
Sources & references
- NCCN — Ovarian Cancer Guidelines · accessed 2026-05-30T00:00:00.000Z
- American Cancer Society — Ovarian Cancer Tumor Markers · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — CA-125 · accessed 2026-05-30T00:00:00.000Z
- ASCO — Ovarian Cancer Tumor Markers · accessed 2026-05-30T00:00:00.000Z
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