What Does CA-125 Mean on a Blood Test?

This page is for anyone who has seen a CA-125 result on a blood test report and wants to know what it means. It covers what the test measures, what it is used for, why it is not a screening test, and how to read a trend.

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. These are 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 explains why the test is so often confusing. Conditions that are not cancer can also raise CA-125, including a normal menstrual period.

The most common use is to monitor someone who has already been 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, MedlinePlus lists people with a strong family history or inherited risk. Their care team may use CA-125 as part of a broader plan. That group is narrow.

Two very large trials settled this question.

The PLCO trial screened 39,105 United States women aged 55 to 74. A comparison group 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 (middle) follow-up of 17 years, NCI describes the evidence as solid that this screening does not lower ovarian cancer deaths.

UKCTOCS tested two approaches. One was ultrasound alone. The other was a multimodal (combined) method: CA-125, interpreted by the Risk of Ovarian Cancer Algorithm, along with ultrasound. Median follow-up was 16.3 years. Neither approach lowered deaths from ovarian cancer, which was the main result the trial measured.

Screening did not just fail to help. It also 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 (the grade used when harms outweigh benefits), 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. Two terms help here:

• Sensitivity is how often a test correctly flags people who have the disease.
• Specificity is how often a test correctly clears people who do not.

The study set 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.

Read plainly: even at that lower cutoff, CA-125 missed about 44% of early-stage cases (100% minus 56%). It misses early cancers often.

The test can be wrong in two opposite ways, 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, on its own, tells you 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 (cancer that starts in the tissue covering the ovary), the most common type, a marker called HE4 is sometimes measured alongside CA-125. NCI lists HE4 for monitoring treatment and checking for recurrence.

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