SAN ANTONIO ? A genetic profiling test already on the market accurately predicts which breast cancer patients will benefit from chemotherapy and which won’t, giving women a powerful tool to help decide whether to undergo the ordeal, scientists report.

The test could help about 80,000 women a year make better choices about how to treat their disease, said federal officials who helped fund the study and conduct it. Half could safely skip chemo, they said.

“It outperformed all known prognostic factors for predicting response,” said Dr. Soonmyung Paik, a University of Pittsburgh physician who presented the research Friday at a breast cancer conference in Texas. Part of it also was published online by the New England Journal of Medicine.

Most breast cancer cases that haven’t spread to lymph nodes will not recur after surgery, but doctors have no good ways to predict which ones will. They guess, based on the size of a tumor and the woman’s age.

The new test, made by California-based Genomic Health, measures the activity of 21 genes to classify women as high, low or medium risk for recurrence.

Previous studies have established the test’s ability to predict this. With this latest study, doctors extended those findings to show that chemotherapy helps high-risk women a lot and low-risk ones not at all.