Fewer than half of Americans have had an AIDS test since guidelines were expanded to include routine screening, according to a government report released Tuesday.

Last year, an estimated 45 percent of Americans ages 18 to 64 reported they've had an HIV test at least once in their lives, up from 40 percent in 2006. That's an increase of 11 million people to 83 million people who have ever been screened, the Centers for Disease Control and Prevention reported in Atlanta.

CDC director Dr. Thomas Frieden said the increase was significant and encouraging, while one outside expert called it disappointing.

"The numbers show that progress is possible. They also show how much more progress is needed," Frieden said during a teleconference.

In 2006, the CDC urged routine testing for everyone ages 13 to 64, even if they're not in high-risk groups. For those at high risk, including gay men and intravenous drug users, annual testing is recommended.

Because more people are getting tested, Frieden said, fewer people are being diagnosed late with HIV. In 2007, about a third of infections were discovered late, an improvement from 37 percent diagnosed earlier in the decade. Frieden said those cases are often only detected when the disease has progressed to AIDS.

AIDS drugs lower the amount of virus and are more effective when given earlier. People who know they are infected are more likely to take steps to prevent spreading it, Frieden said.

He said 28 percent of those at high risk have never been tested.

"If you don't know your HIV status, you can't effectively protect yourself and your partners," he said.

The CDC estimates that about 1.1 million Americans have HIV, but about 20 percent don't know it. About 56,000 new infections occur each year in the U.S.

The CDC report was based on nationwide health surveys and state reports on infections.

Dr. John Bartlett of Johns Hopkins University called the testing figures disappointing.

"It's an incremental gain," said Bartlett, an infectious disease specialist.

He said that when the CDC changed its guidelines, many states still had laws that required special counseling before and after HIV tests. Most have since dropped the restrictions, but there are still some barriers. Maryland, for example, still requires doctors to note the patient's consent on charts, he said.

He said more screening could be done if hospitals were pressured to adopt routine HIV testing of patients by Medicare or hospital groups.