What Is the CA-125 Test?

The cancer antigen 125 (CA-125) test is a laboratory study done to evaluate the concentration of CA-125 in the blood. This molecule is considered a tumor marker because an elevated concentration in the blood can signal the presence of an underlying cancer. Although high levels are most closely associated with ovarian cancer, they can also be caused by other tumors and diseases. The test is not an ideal screening test for ovarian cancer, but does have an important role in detecting ovarian cancer recurrence in select patients.

CA-125 often serves as a marker that cancer is present in the body. Malignant growths, including ovarian cancer, produce this substance and shed it into the bloodstream. Concentrations in the blood greater than 35 units per milliliter are suspicious for the presence of underlying cancers. Approximately 80% of women with ovarian cancer have a level exceeding this threshold.

As mentioned above, elevations in CA-125 are most closely associated with ovarian cancer. Other cancers, including those arising from the breasts, lungs, pancreas, and uterus, can also cause high concentrations of this substance. It is generally considered to be a marker of epithelial cancer, which is a category of malignancies that arise from cells lining different organs of the body.

Although elevations in CA-125 are most commonly seen in people with cancer, other conditions can also cause high levels. Patients with diseases such as endometriosis, inflammatory bowel disease, benign ovarian cysts, pelvic inflammatory disease, and cirrhosis of the liver can have elevated levels of this substance. Some women also have variations in blood concentration of this molecule throughout their menstrual cycles.

Many researchers have tried to use the CA-125 blood test as a way to screen for the presence of ovarian cancer, but this practice has not become widespread for a number of reasons. First, not all women with ovarian cancer have abnormalities on this blood test. Using the test for screening purposes could fail to identify over 20% of women with ovarian cancer. Second, elevations in the test can be caused by non-malignant conditions. Women found to have elevations in the test for benign reasons could be subjected to the stress of a possible cancer diagnosis, and would also have to endure the expense and inconvenience of additional, more thorough testing for ovarian cancer.

Measuring CA-125 does play an important role in monitoring women who are recovering from ovarian cancer. After being treated with surgeries, radiation, and chemotherapy to eradicate the malignancy, patients might be in remission and not display any clinical evidence of lingering disease. Later regrowth of cancerous cells can cause an increase in the CA-125 level, signaling that the cancer might be recurring.