The AIDS virus can remain undetected within the human body for more than a year, far longer than medical experts had thought possible, research indicates.

The research suggests that those infected with the AIDS virus for many months may still show negative results from widely used tests.

For thousands who have taken the AIDS-antibody test in the last two years, negative test results may have been premature. More specific confirmation may be necessary, the study suggests.

"If people think the latency period is very short, they may be wrong," said Genoveffa Franchini, a National Cancer Institute (NCI) researcher and an author of the study published in The Lancet, the British medical journal.

"The results surprised us. What it means is clear. The period before the development of antibodies is longer than anyone thought. But we still don't know how long people are infected with this disease before it appears on tests," she said.

Scientists had thought that antibodies to the virus usually develop between three and 12 weeks after infection. The recent study, by researchers from the NCI and Finland, shows that, in some cases, individuals do not develop antibodies until 14 months after infection.

The results could have significant implications for the efficacy of any system of widespread, routine testing or the tracing of sex partners. But, even if the latency period is much longer than currently known, it would not be likely to increase the risk to the blood supply. Blood bank officials said screening procedures make it rare for people at high risk for AIDS to donate blood.

"The safety of blood donors is the only graph of data I have ever seen related to AIDS that is going down," said Dr. Gerald Sandler, associate vice president for medicine of the American Red Cross. "Each month, it is getting safer in this country to have a blood transfusion."

While some infected donors will always slip through detection procedures, he said, the number has fallen steadily since introduction of AIDS blood tests in 1985.

Only one in 10,000 donors is believed to pass on the AIDS virus without detection, medical authorities say. According to the Red Cross, approximately 75 people a year will contract the disease through transfusions.

Scientists do not know how much HIV, the virus that causes AIDS, is necessary to infect a person. But the new study of sexually active gay men showed evidence of latent infection in nine of 25 men who did not show positive results on conventional tests.

Using a new generation of specific tests, however, scientists identified the very beginnings of the infection as many as 14 months before the men showed antibodies to HIV.

The study indicates it may be necessary to use more specific and expensive tests than are commonly employed to detect a latent HIV infection. But it does not help to determine how likely an infected person is to transmit AIDS or how much antigen in blood will do so.

Currently, AIDS testing involves two procedures, done sequentially. The first, known as ELISA, is sensitive enough to detect almost all infected people, but it also shows a high rate of uninfected people falsely testing positive.

If the first ELISA is positive, it is usually repeated for verification. If the second is also positive, the Western blot is used.

But in the new study, researchers used a more specific antigen test that can identify several proteins made by the AIDS virus. Antigen tests are more accurate because they actually indentify proteins in the virus. Antibody tests simply measure the body's response to infection.

"There are many unanswered questions," Franchini said. "We don't know how many sexual contacts these people have had, and we don't know what other factors could have contributed to their infection."

The study was conducted in Finland. And Franchini said large-scale U.S. tests will be necessary to confirm the findings of long-term latency. It is important to develop a test to measure how much virus is needed to infect cells, she added.

"The implications are still uncertain. We just don't know where it will lead, yet," she said.