But a new study by Professor David Holtgrave (Johns Hopkins Bloomberg School of Public Health) in this week's PLoS Medicine finds that testing only those people at high risk of HIV, rather than offering everybody a test, would be a better strategy for diagnosing HIV infections and for helping to stop the spread of the virus.

About a quarter of a million people in the US do not realize they are infected with HIV. Because they are unaware of their infection, they don't get the medicines they need to stay healthy, and they may also be transmitting HIV to others unwittingly. The CDC hopes that its new recommendations on opt-out testing will allow health professionals to reach more of these people.

In order to assess the potential costs and public health impact of the CDC recommendations, Professor Holtgrave used two standard research techniques (cost effectiveness analysis and scenario analysis) to compare opt-out testing with "targeted counseling and testing." In targeted testing, health professionals focus their efforts on those who are at increased risk of being HIV positive (e.g. patients attending sexual health clinics, drug treatment centers, or clinics in prisons). Such patients are offered counseling before the test, to assess their actual risk of HIV and to discuss what would happen in the event that the HIV test comes back positive. During counseling, people are also given advice on steps they can take to stay HIV negative if their test comes back negative, and to prevent infecting others if their test comes back positive.

According to Professor Holtgrave's analysis, opt-out testing might reach 23% of those people who are currently unaware that they are HIV positive.The program might also prevent 9% of the 40,000 new HIV infections that occur each year in the US. The cost of averting one new infection would be US$237,149. In contrast, targeted counseling and testing might identify about 75% of people in the US now unaware they are living with HIV infection, and prevent about 36% of the new HIV infections. The cost of averting one new infection would be US$59,383.

Even when the author changed several assumptions in his analysis (e.g., assumptions about levels of HIV infection or the effectiveness of counseling), he found that targeted counseling and testing still performed better (so the results are ''robust'' across a variety of such assumptions). -Public Library of Science