The cancer antigen 125 test returns a false positive if medical personnel administer it at certain times during the menstrual cycle. Pelvic inflammatory disease, liver disease, pregnancy, lupus or endometriosis can also cause a false positive, according to WebMD.
The CA-125 test is unable to determine the difference between benign lumps and cancerous tumors, according to WebMD. While labs normally test the blood to determine the antigen, they also test other body fluids from the chest or stomach area for the antigen. As of 2015, there is not enough evidence to show the CA-125 test is effective as a method of early ovarian cancer discovery.
CA-125 is a protein from the surface of many ovarian cancer cells, according to WebMD. The CA-125 test measures the amount of the antigen in the blood. However, CA-125 also collects on other types of cancer cells and some normal tissues.
Doctors use the CA-125 to determine the effectiveness of cancer treatment. If the treatment is working, the level of the antigen should decrease, according to WebMD. In ovarian cancer survivors, the CA-125 level often increases months before the return of the cancer is detectable in any other way. In women with cancer, a high CA-125 level indicates the cancer likely started in the ovary.