What Does CA-125 Mean on a Blood Test?

CA-125 stands for cancer antigen 125. It is a protein, and a blood test measures how much of it is circulating.

It belongs to a family of tests called tumor markers — substances made either by cancer cells, or by normal cells reacting to cancer. NCI's tumor marker list gives CA-125 one cancer type, ovarian cancer, and three uses: helping in diagnosis, assessing response to treatment, and evaluating recurrence.

MedlinePlus adds the biology that explains most of the confusion around it. Organs in the female reproductive system, including the ovaries and fallopian tubes, can make CA-125 when they are inflamed. That inflammation can come from a normal menstrual period, or from cancer, or from several conditions in between.

The most common use is straightforward monitoring in someone already diagnosed.

• During treatment. Levels falling across several tests usually means treatment is helping. Levels rising or staying flat may mean it is not.
• After treatment. Levels climbing over time may mean the cancer is returning.
• To help evaluate a pelvic mass. If imaging shows a suspicious lump, CA-125 is measured alongside other tests. It does not settle the question. Only a biopsy diagnoses ovarian cancer.
• In people at very high risk. For someone with a strong family history or an inherited risk, a clinician may check CA-125 as part of a broader plan.

That last category is narrow. High risk means things like a mother or sister with ovarian cancer, two or more other affected relatives, a BRCA1 or BRCA2 change, or Lynch syndrome.

This is the part worth understanding in detail, because two very large trials settled it.

The PLCO trial enrolled 78,216 United States women aged 55 to 74 at ten centers between November 1993 and July 2001. Half were screened, half received usual care. Screening was six annual CA-125 tests, using 35 U/mL as the threshold for a positive result, plus four annual transvaginal ultrasounds. After a median follow-up of 17 years, NCI describes the evidence as solid that this screening does not lower ovarian cancer deaths.

UKCTOCS tested two approaches, ultrasound alone and a multimodal method using CA-125 interpreted by the Risk of Ovarian Cancer Algorithm along with ultrasound. Women had 7 to 11 screens and a median of 16.3 years of follow-up. Neither approach showed a mortality benefit on the primary endpoint.

Screening was not simply useless. It caused harm:

• In PLCO, 9.6% of screened women had a false-positive result, and 6.2% went on to surgery. The surgical complication rate was 1.2% across all screened women.
• Minor complications ran 58.3 per 10,000 women screened with CA-125, against 3.3 per 10,000 screened with ultrasound.
• In UKCTOCS, the ultrasound-only arm produced 50 false-positive surgical procedures per 10,000 screens. The multimodal arm produced 14.

That evidence is why the U.S. Preventive Services Task Force recommends against screening for ovarian cancer in women without symptoms. It is a grade D recommendation, and it applies to women not known to carry a high-risk hereditary cancer syndrome.

The most commonly used cutoff for a positive result is 35 U/mL. That was the value used in PLCO. It is a convention, not a biological boundary.

A biomarker study measured how well CA-125 actually detects disease. Setting the specificity at a fixed 95%, the threshold that produced it was 24 U/mL — lower than the clinical cutoff. At that setting:

• Sensitivity for early-stage disease, meaning stage I and II, was 56%, with a confidence interval of 49% to 72%.
• Sensitivity across all cases was 73%, with a confidence interval of 64% to 84%. In that group, 56% already had stage III or IV disease.

When the clinical cutpoint of 35 U/mL was used instead, sensitivity fell.

Read plainly: at the threshold used in practice, CA-125 misses roughly half of early ovarian cancers. That is the single most important fact about this test.

Two failure modes run in opposite directions, and both matter.

High without cancer. MedlinePlus names endometriosis, pelvic inflammatory disease, uterine fibroids, and liver disease. A menstrual period and pregnancy can also raise it.

Normal with cancer. Levels can be low in early stages, and not everyone with ovarian cancer produces high CA-125 at all.

So a single number, in isolation, answers very little. Your clinician reads it against your imaging, your symptoms, your history, and your previous values.

In monitoring, the shape of the line matters more than any one point. A stable level across several tests is generally reassuring. A rising trend usually prompts a closer look with imaging.

Questions worth asking:

• What was my level, and what was it last time?
• Is the direction changing, or is this within normal variation?
• Was the sample taken at a point in my cycle that could affect it?
• Does anything besides cancer explain a change in my case?

One more test may come up. For epithelial ovarian cancer, the most common type, a marker called HE4 is sometimes measured alongside CA-125. Some studies suggest the pair gives more accurate information about whether treatment is working and whether the cancer is returning.

If you are trying to understand other terms on a laboratory or pathology report, our page on pathology reports walks through the structure. For how markers work as a class, see tumor markers. For the disease this test tracks, see ovarian cancer.

• National Cancer Institute, Tumor Markers in Common Use, accessed August 6, 2026
• National Cancer Institute, Ovarian, Fallopian Tube, and Primary Peritoneal Cancers Screening PDQ, health professional version, updated April 9, 2025; accessed August 6, 2026
• MedlinePlus, CA-125 Blood Test (Ovarian Cancer), accessed August 6, 2026
• U.S. Preventive Services Task Force, Ovarian Cancer: Screening, accessed August 6, 2026