CA 125 test: what it shows and how the result is read

CA 125 (cancer antigen 125) is a protein produced by the tissues of the female reproductive organs, especially when those tissues are inflamed or irritated. The body can also produce a raised amount of CA 125 in response to ovarian cancer, which is why this indicator is classed as a tumour marker. The CA 125 test is most often used not for making the first diagnosis, but to follow how well treatment is working in ovarian cancer that has already been diagnosed, and to notice a possible recurrence after treatment. Less often it is used to assess a suspicious pelvic mass, or to monitor people at high risk.

CA 125 is not recommended as a screening test for women who have no symptoms, because the indicator is not specific enough: non-malignant conditions can also raise it — endometriosis, pelvic inflammatory disease, a uterine fibroid, liver disease, and also menstruation or pregnancy. In addition, in the early stages of cancer CA 125 can stay within the normal range, so a normal result does not rule out the possibility of disease.

• To follow how well treatment is working in ovarian cancer that has already been diagnosed
• After treatment has ended — to check whether CA 125 is rising, which could indicate that the disease has come back
• When a suspicious pelvic mass (a mass in the lesser pelvis) has been found on ultrasound or another test
• To assess women at high risk (for example with an inherited cancer risk), together with other tests
• When there are symptoms that raise a suspicion of ovarian disease, together with imaging tests

A raised CA 125 is most often assessed in the context of ovarian cancer that has already been diagnosed — it can show whether the treatment is working, or whether the disease has possibly come back. However, CA 125 also rises for non-malignant reasons: endometriosis, pelvic inflammation, a uterine fibroid, liver disease, and also during menstruation or pregnancy. A raised CA 125 on its own therefore does not mean cancer.

• Ovarian or another cancer — CA 125 is used to follow the response to treatment and a possible recurrence
• Endometriosis
• Pelvic inflammatory disease (PID)
• Uterine fibroid
• Liver disease
• Natural physiological changes — menstruation, pregnancy

What to discuss with a doctor

• Whether CA 125 is rising or falling over the course of treatment, rather than a single separate value
• Additional imaging tests (ultrasound and others), if a pelvic mass is suspected
• Whether the phase of the menstrual cycle, pregnancy or a known non-malignant condition (endometriosis, a fibroid) could have had an effect
• In high-risk cases — HE4 or the ROMA index assessed alongside

Why CA 125 is not a screening test

According to MedlinePlus, CA 125 is not recommended as a screening test for women who have no symptoms, because the indicator is not specific enough and can be raised for many non-malignant reasons. It works the other way round as well: in the early stages of cancer the value can stay within the normal range, so a normal CA 125 does not rule out the possibility of disease, particularly when there are other features that raise suspicion. Which tests make sense in a particular situation is decided by a doctor.

Related pages: HE4, the ROMA index, CA 15-3, CA 19-9, AFP, how to prepare for a blood test and how to read blood test results.