Routine HIV screening at a Colorado hospital identified only six more new cases than standard, targeted testing, according to a two-year study.
By comparison, an estimated 230,000 Americans currently have undiagnosed HIV, researchers report in the Journal of the American Medical Association.
Today, many doctors only offer testing to patients they deem at risk, such as prostitutes or drug addicts. But since 2006, the U.S. Centers for Disease Control and Prevention have recommended large-scale screening regardless of risk factors in areas with many undiagnosed cases.
The hope is that identifying people with HIV would allow treatment to begin earlier and help curb new infections.
"There is no question that it would have an effect on prevention," said Dr. Bernard M. Branson of the CDC, who was not involved in the new study. He said people are less likely to transmit the disease when they know they have it, and that drug treatment has the added benefit of making people much less infectious.
Currently, about one million Americans are HIV positive.
In the new study, Dr. Jason Haukoos at the Denver Health Medical Center and colleagues found that offering opt-out testing to patients in the emergency department did increase the number of identified HIV infections. But not much.
Only about a quarter of more than 28,000 patients accepted the test, and of these, only 10 had new HIV diagnoses.
When doctors relied on standard risk factors in a patient group about the same size, they found 4 people with undiagnosed HIV among 243 tested patients.
"In some places it may be that the yield of screening is too low," said Branson, although he stressed that wasn't necessarily the case in Denver.
With three-fourths of the patients opting out of the test, he said, "the question is whether screening isn't worth doing or you need to do a better job of reaching more people."
In an editorial, Dr. Roland C. Merchant and Dr. Michael J. Waxman of Brown University in Providence, Rhode Island, noted that "patients often decline testing based on the belief that they are not at risk for HIV, even if they are."
"The disconnect between perceived and actual risk is important and may reflect a patient's fear of coming to terms with risk and lack of knowledge about HIV transmission," they added.
In other parts of the country, such as Washington, D.C., or Oakland, Calif., Branson said people appeared to be more willing to be tested.
The cost of testing varies with the specific method. Branson said one study had estimated the cost at $77 per patient with methods comparable to those used by the Colorado team, while an automated system could bring it down to $12.
But, he said, "the lifetime cost of therapy for a person who is infected is obviously high, so you don't have to prevent very many cases to make screening worthwhile."
The U.S. Preventive Services Task Force, a federal expert panel, currently doesn't have specific recommendations about universal HIV screening.