Atlanta ? Federal health officials said Thursday they were confident about the accuracy of the main antibody test being used to screen people for SARS.

The Centers for Disease Control and Prevention administered the test to more than 500 people — some suspected of having SARS, some known not to be infected — and said no one tested positive for the disease who did not have it.

“It’s a good test,” said Dr. Larry Anderson, a CDC virus expert. “I think it’s going to help a lot on understanding SARS.”

Scientists still need to develop a test that can detect infection earlier, Anderson said. Sometimes the antibody test does not detect the virus until four to six weeks after a person shows symptoms.

The CDC tested nearly half of the 66 probable SARS cases in the United States. Only seven people — about 22 percent — had the antibody. That did not surprise officials, because many probable SARS patients may actually have a different respiratory illness.