What the CA-125 test shows
CA-125 (cancer antigen 125) is a large mucin glycoprotein — the product of the MUC16 gene — shed into the blood from tissues with a shared embryonic origin: the lining of the ovaries and fallopian tubes, the endometrium, and the membranes of the abdomen and chest (peritoneum, pleura, pericardium). When one of these surfaces is inflamed, stretched or invaded, more CA-125 reaches the blood. MedlinePlus calls it a tumor marker most closely linked to the female reproductive tract.
Its greatest value is not in finding cancer but in watching one that is already known. Per the National Cancer Institute, CA-125 is used mainly to judge whether ovarian-cancer treatment is working and to catch a recurrence, by following the trend when the level was high at diagnosis.
It differs from the markers it is ordered with. HE4 is more specific to ovarian cancer and less easily raised by benign conditions, so the two are combined to weigh up a pelvic mass. CEA and CA 19-9 lean toward gastrointestinal and mucinous tumors; pairing them with CA-125 helps tell a primary ovarian cancer from a bowel or stomach cancer that has spread to the ovary.
CA-125 normal range
CA-125 is reported in U/mL. The SI unit, kU/L, is numerically identical (1 U/mL = 1 kU/L), and some labs write IU/mL — the number is the same. The long-standing upper limit is 35 U/mL, set at roughly the 99th percentile in healthy women.
| Group | Orientation, U/mL (= kU/L) |
|---|---|
| Conventional cutoff (adults) | < 35 |
| Premenopausal women | often < 35, but benign rises are common |
| Postmenopausal women | usually lower; a value > 35 carries more weight |
| Pregnancy (first trimester) | can be transiently raised |
| Men and children | low; rarely measured |
The trend matters more than any single reading. In the UK, NICE long advised that a woman with symptoms of ovarian cancer and a CA-125 of 35 U/mL or more be referred for a pelvic ultrasound; in 2026 it replaced that single cutoff with age-specific thresholds — lower in older women, and not used on its own under 40 — since the same number means different risk at 45 and 70. Ranges depend on the lab, assay and menopausal status — read your result against your own report.
Why CA-125 is high
A raised CA-125 is common and, most of the time, is not cancer — especially before menopause. Roughly by frequency:
- Benign gynecological conditions (commonest, premenopausal). Endometriosis, uterine fibroids, adenomyosis, ovarian cysts, pelvic inflammatory disease, an ordinary menstrual period and normal early pregnancy can all raise it, as StatPearls notes.
- Irritation of the abdominal or chest lining. The peritoneum and pleura make CA-125, so cirrhosis with ascites, heart failure, pancreatitis, diverticulitis, recent abdominal surgery and fluid around the lungs can all lift it.
- Gynecological cancers. Epithelial ovarian cancer — particularly high-grade serous — plus fallopian-tube, primary peritoneal and endometrial cancers.
- Other cancers. Pancreatic, lung, breast, colon and stomach cancers can raise it too.
How worrying a high value is depends on context: far more often benign before menopause, but carrying more weight after it, especially with a pelvic mass or a rising trend.
When is it urgent? A postmenopausal woman with a pelvic mass and a raised CA-125 needs prompt referral to a gynecologist or gynecologic oncologist, and during cancer follow-up a clearly rising level — often the first sign of recurrence — should be reported without delay.
Why CA-125 is low
A low or normal CA-125 — under 35 U/mL — is the reassuring, expected state; there is no “deficiency” to correct. Two situations still deserve care.
First, a normal result does not rule out ovarian cancer. Only about half of early-stage ovarian cancers raise CA-125, and mucinous and some clear-cell tumors often leave it normal, per StatPearls. Anyone with a suspicious mass or persistent symptoms such as bloating or pelvic pain still needs imaging and follow-up even when the number is normal.
Second, in someone under treatment a falling CA-125 is the good direction: it signals that chemotherapy is shrinking the tumor, and a return into the normal range is a goal of treatment.
Together, these are why CA-125 is a poor screening test. The USPSTF recommends against screening for ovarian cancer in women at average risk, because it does not lower deaths and leads to false alarms and needless surgery.
What to test alongside
CA-125 is read alongside other tests, not on its own:
- HE4 — combined with CA-125 in the ROMA score to gauge whether a pelvic mass is malignant; more specific, and less raised by endometriosis.
- CEA — helps separate a primary ovarian cancer (CA-125 high, CEA usually normal) from a bowel or stomach cancer that has spread.
- CA 19-9 — often raised in mucinous ovarian tumors and pancreatic cancer, which CA-125 may miss.
- AFP — with a pelvic mass in a younger woman, points toward a germ-cell tumor.
- CA 15-3 — a breast-cancer marker that can co-rise when the primary site is unclear.
- ALT and AST — liver checks; cirrhosis is a classic benign reason for a raised CA-125.
What to do about an abnormal result
- Don’t panic or self-diagnose. A single raised CA-125 is far more often benign than cancer, especially before menopause.
- Look at the trend, not one number. An isolated value means little; results are usually repeated, and the direction of change is what counts.
- For a raised CA-125, the usual next step is a pelvic or transvaginal ultrasound, plus a look for benign explanations — period timing, endometriosis, fibroids, pregnancy, liver or heart disease. Start with your GP or a gynecologist.
- A postmenopausal woman with a pelvic mass and a raised CA-125 should be referred to a gynecologic oncologist; risk scores such as ROMA and RMI guide that decision.
- Don’t use CA-125 to screen yourself if you are healthy and at average risk. With a strong family history or a known BRCA change, discuss a tailored plan with your doctor rather than relying on the test alone.
- See your GP or gynecologist first — they arrange the imaging and coordinate any referral rather than jumping to treatment.
Mini-FAQ
What is a normal CA-125 level?
Below 35 U/mL is the usual cutoff, but the trend over time matters more than one reading, and ranges vary by lab and by menopausal status.
Does a high CA-125 mean I have ovarian cancer?
No. Many benign conditions — endometriosis, fibroids, pelvic infection, menstruation, pregnancy, and liver or heart problems — raise it, especially before menopause, so a high value is more often benign than cancer.
Can CA-125 be normal if I have ovarian cancer?
Yes. Only about half of early-stage ovarian cancers raise CA-125, and some types such as mucinous and clear-cell tumors often do not, so a normal result does not rule cancer out.
Is CA-125 a good screening test for healthy women?
No. The USPSTF recommends against ovarian cancer screening in average-risk women, because it does not reduce deaths and leads to false alarms and unnecessary surgery.
What is CA-125 actually used for?
Mainly to monitor a known ovarian cancer — checking whether treatment is working and watching for recurrence — and, alongside ultrasound and HE4, to help assess a pelvic mass.
