"Although the antibody test can deliver immediate results, it is not likely to identify an infection that has occurred in the past month. The genetic test, which looks for bits of the virus’s ribonucleic acid, or RNA, can identify infections in a week to 10 days. Chris, who asked that his full name not be used, was told that his antibody test was negative. But his RNA test was positive, making him one of the few whose diagnosis came in the acute, or primary, phase of infection, when the virus is replicating aggressively but the body has not yet mounted an effective immune response. "