The Food and Drug Administration recently approved a new blood test that can detect the presence of residual ovarian cancer cells in the body after the main tumor is surgically removed.

"The CA 125 assay is the first and only blood test of its kind," Centocor Inc. of Malvern, Pa., said in its announcement.

The test, one of the first of what is expected to become a large battery of cancer tests, relies on a monoclonal antibody that reacts with protein markers on the surface of ovarian cancer cells. By tagging the monoclonal antibody with a radioactive tracer, it is possible to determine how many ovarian cancer cells remain in the body.

The test could be used to detect ovarian cancer earlier than current tests, monitor the effectivness of a therapy by detecting changes in the number of cancer cells in the body, and differentiate cancer from noncancerous tumors of the ovary.

Ovarian cancer, which most often occurs after menopause, is the fourth leading cancer killer of women in the United States. Some 18,500 cases are diagnosed each year. The cure rate remains low -- only 30 percent can be considered cured -- and this cancer kills more than 12,000 women a year.

Ovarian cancer, which has no early symptoms, is usually diagnosed after it is well developed. Traditional treatment approaches -- primarily surgery but also radiation therapy and chemotherapy -- are often ineffective.

In addition to using the CA 125 as a diagnostic test, Centocor hopes to use the antibody as a therapy to track down and kill widespread cancer cells, just as a guided missile is able to track down and destroy its target.