💡 In one sentence: CA-125 is a protein found on the surface of many ovarian cancer cells — but it is also elevated in many benign conditions, making it a monitoring tool rather than a definitive cancer test.
What Does CA-125 Measure?
CA-125 is a glycoprotein produced by cells lining the abdomen and pelvis (mesothelial cells) and some reproductive organs. It is elevated in the majority of women with epithelial ovarian cancer — particularly the serous subtype. However, it is also elevated in many benign conditions: endometriosis (commonly), uterine fibroids, pelvic inflammatory disease, liver disease, pleural effusion, and normal menstruation. It is also elevated in other cancers (fallopian tube, endometrial, peritoneal). Because of its low specificity, CA-125 is not recommended as a standalone screening test in the general population — it is most useful for monitoring known ovarian cancer treatment response and detecting recurrence.
Understanding Abnormal Results
⬆️ High CA-125 may indicate:
Mildly elevated (35–100) with normal scanMost likely benign (endometriosis, fibroids) — review with clinical context; repeat in 4–6 weeks or after menstruation
Moderately elevated (100–500) with complex ovarian cystHigh enough to significantly increase cancer probability alongside a suspicious scan — gynaecological oncology referral
>500 U/mLVery high concern for ovarian cancer — CT staging scan and urgent specialist referral
Frequently Asked Questions
Is CA-125 a reliable cancer test?
No — as a standalone screening test, CA-125 is not reliable. Its specificity for ovarian cancer is low because it is elevated by so many benign conditions. In pre-menopausal women, endometriosis and fibroids commonly cause CA-125 above 35 U/mL without any cancer. In post-menopausal women, where benign causes are less common, an elevated CA-125 with a complex ovarian mass is more concerning. CA-125 is most reliable when: used in post-menopausal women, combined with a transvaginal ultrasound, and expressed as a Risk of Malignancy Index (RMI).
My CA-125 is 78 with endometriosis — does this mean cancer?
Almost certainly not. Endometriosis is one of the most common causes of elevated CA-125 — often above the 35 U/mL threshold, sometimes above 100 U/mL. In a pre-menopausal woman with a known endometriosis diagnosis, a CA-125 of 78 is expected and does not indicate cancer. It may be useful for monitoring endometriosis activity (a rising CA-125 can indicate progression), but it should not trigger cancer anxiety.
Why is my oncologist monitoring my CA-125 after ovarian cancer treatment?
CA-125 is an excellent monitoring marker once ovarian cancer has been established and treated. After surgery and chemotherapy successfully treat ovarian cancer, CA-125 typically falls to within the normal range. A subsequent rise from this low baseline — even if still within the "normal" range — can be the first sign of cancer recurrence, often appearing months before symptoms or imaging changes. This early warning allows prompt treatment initiation, which improves outcomes.
Should I request a CA-125 test for general cancer screening?
No — national and international oncology guidelines do not recommend routine CA-125 screening for ovarian cancer in the general population because it generates too many false positives (leading to unnecessary surgery) without reliably detecting early-stage disease. The exception is women with BRCA1/BRCA2 gene mutations or a strong family history of ovarian cancer — in whom surveillance (CA-125 + transvaginal scan) is offered, though even this remains imperfect.