CA-125 is a biomarker, a protein called Cancer Antigen 125, that may be elevated in the blood of those with ovarian cancer. As a tool for ovarian cancer detection (or screening), it is not perfect – CA-125 is also often elevated in the blood of people with endometriosis, fibroids, liver cirrhosis, and who are pregnant (just to name a few conditions). Furthermore, it misses about half of actual ovarian cancer cases, and therefore ovarian cancer screening guidelines do not recommend the CA-125 blood test as a screening tool in the general population. When doctors suspect ovarian cancer, they often measure CA-125 levels in their patients, but the test is just one of the tools in the doctor's toolbox. CA-125 is FDA approved for assessing treatment effectiveness after diagnosis, as well as monitoring for recurrence.  The only way to definitively diagnose ovarian cancer is through surgery. And since CA-125 levels are often elevated for any number of reasons, using that blood test as a screening tool would send otherwise healthy people into unnecessary surgery. In fact, this could cause more harm (both emotional and physical) to more people than it would save lives, as the exploratory surgery itself comes with risk.