CA-125 measures the level of a specific protein called MUC16 in your blood. This protein sits on the surface of cells lining your organs, including the ovaries, uterus, and lungs. The normal reference range is 0 to 35 units per milliliter (U/mL). Levels above that threshold can signal ovarian cancer, but they also rise in a number of noncancerous conditions, which makes the test useful in some situations and misleading in others.
What CA-125 Actually Is
CA-125 stands for cancer antigen 125. The protein it detects, MUC16, belongs to a family of large, sugar-coated molecules called mucins. These mucins coat the surfaces of your organs and act as a protective barrier, shielding cells from pathogens and physical damage. MUC16 is normally tethered to cell membranes in the lining of the reproductive tract, lungs, and eyes.
When cells producing MUC16 are damaged, inflamed, or growing abnormally, they shed more of this protein into the bloodstream. A simple blood draw captures that circulating protein, and the result is reported in units per milliliter. Values up to 35 U/mL are considered normal. Borderline results fall between 35 and 65 U/mL, while readings above 200 U/mL are unlikely to come from a benign cause.
Its Primary Role in Ovarian Cancer
CA-125 is most closely associated with ovarian cancer, but its usefulness depends heavily on the stage of disease. In early-stage ovarian cancer, only about 50 to 62 percent of patients have elevated levels. That means roughly half of early cases would be missed if this test were used alone. In advanced ovarian cancer, the picture changes dramatically: 92 percent of patients show elevated CA-125.
This gap between early and late detection is a major limitation. The U.S. Preventive Services Task Force recommends against using CA-125 to screen for ovarian cancer in women who have no symptoms and no known high-risk hereditary cancer syndrome. The reasoning: screening with CA-125, transvaginal ultrasound, or both has not been shown to reduce ovarian cancer deaths. False positives, on the other hand, can lead to unnecessary surgeries and the complications that come with them. The task force concluded that the harms of routine screening outweigh the benefits.
That said, CA-125 remains valuable once ovarian cancer is already suspected or diagnosed. It plays a central role in monitoring how well treatment is working and watching for recurrence after treatment ends.
How It Tracks Treatment Response
For patients already diagnosed with ovarian cancer, serial CA-125 measurements during chemotherapy provide a reliable signal of whether treatment is working. Clinicians look for specific patterns in the numbers over time rather than relying on a single reading.
A 50 percent drop in CA-125 across two consecutive samples, confirmed by a fourth sample taken at least 28 days later, is considered a partial response. A 75 percent decline over three consecutive samples meets a stricter definition of response. These criteria have proven highly reliable: in a study of over 600 patients, only 0.3 percent showed a CA-125 response while their cancer was actually progressing. Rising levels after a period of decline can be an early warning of recurrence, sometimes months before symptoms appear or imaging picks up new tumors.
Other Cancers That Raise CA-125
Ovarian cancer gets the most attention, but CA-125 is not specific to the ovaries. Nearly half of patients diagnosed with pancreatic cancer and lung cancer in one large population-based study had CA-125 levels at or above 35 U/mL. The test can also be elevated in endometrial, breast, and gastrointestinal cancers. Because of this, clinicians investigating an elevated CA-125 result are encouraged to consider cancers beyond the ovaries, particularly when imaging has ruled out ovarian disease.
Noncancerous Causes of Elevated Levels
A high CA-125 result does not automatically point to cancer. Several common, benign conditions can push levels above the normal range:
- Endometriosis, one of the most frequent noncancerous causes
- Menstruation, particularly during the first few days of a period
- Pregnancy
- Uterine fibroids
- Pelvic inflammatory disease
- Liver disease
This list is one reason the test works poorly as a screening tool in premenopausal women. Menstruation alone can temporarily spike CA-125, and endometriosis affects roughly 10 percent of women of reproductive age. A single elevated reading in these populations is more likely to reflect a benign cause than cancer.
How Your Menstrual Cycle Affects Results
CA-125 levels fluctuate throughout the menstrual cycle. They tend to peak during menstruation, likely because of increased inflammatory activity in the uterine lining. Levels measured on days two through four of a period can be meaningfully higher than levels drawn at mid-cycle, around days 13 to 15. For this reason, if your doctor is using CA-125 to investigate a condition like endometriosis, the timing of the blood draw relative to your cycle matters. In some cases, comparing a menstrual-phase sample to a mid-cycle sample improves diagnostic accuracy. One study found that all patients with a positive mid-cycle CA-125 result were later confirmed to have deep endometriosis.
Combining CA-125 With Other Markers
Because CA-125 alone has clear limitations, it is sometimes paired with a second blood marker called HE4 (human epididymis protein 4). The two values feed into a calculation known as the ROMA score, short for Risk of Ovarian Malignancy Algorithm. ROMA sorts patients into low-risk and high-risk categories for ovarian cancer, and its sensitivity is higher than either marker used independently. This combined approach is particularly useful when a pelvic mass has already been found on imaging and the clinical question is whether it is likely to be cancerous or benign.
ROMA does not replace imaging or biopsy. It adds a layer of information that helps guide decisions about whether a patient needs referral to a gynecologic oncologist or can be managed by a general gynecologist.