RISK OF GETTING HIV-TAINTED BLOOD SMALLER, STUDY SAYS.

Byline: Jason Kahn Medical Tribune News Service

In some reassuring news for those afraid of contracting the AIDS virus through a blood transfusion, a new study shows that the risk of transmission associated with the latest tests used to screen donated blood is even smaller than scientists previously had estimated.

The chances of a single unit of blood slipping through and infecting a recipient with HIV are as small as one per every 660,000 donations, federal investigators reported Thursday in The New England Journal of Medicine.

The researchers based their estimate on a study of 5 million blood donations obtained by the American National Red Cross.

Since the Red Cross collects roughly half of all the blood donated each year in the United States, that means that 18 to 27 of the 12 million donations screened annually harbor the AIDS virus, said lead study author Dr. Eve Lackritz, of the Centers for Disease Control and Prevention in Atlanta.

Current tests for screening blood donations look for antibodies that develop in a person's blood in response to infection with HIV. The problem: There's a so-called "window period" when a recently infected donor is infectious, but HIV antibodies have not yet appeared. During this period, an HIV-tainted unit could escape detection, raising the risk of infecting a recipient, the study authors explained.

With the antibody tests used since 1985 - when HIV screening of all blood donations was introduced in the United States - the estimated window period was 45 days. But due to improvements in the screening tests, scientists have now cut that time period nearly in half, to 25 days, according to the researchers.

Nevertheless, the U.S. Food and Drug Administration and other groups are considering using newer and more sensitive tests aimed at reducing the odds of missing a unit of donated blood during the window period even further, the study authors noted. But because the risk is already extremely small, "new interventions will be of decreasing benefit," they wrote.

The benefits, though, depend on one's point of view, said Dr. Thomas Quinn, a senior investigator at the National Institute of Allergy and Infectious Diseases in Bethesda, Md. "If you're that one in 600,000 who gets infected," then improved screening tests would have been worthwhile regardless of the cost, he said.

Quinn stressed, however, that the new study is "right on the mark" in showing that the risk of contracting HIV infection from a donated unit of blood is extremely low.

In the new study, the researchers analyzed 5 million blood donations made between 1992 and 1993, taking into account the incidence of HIV among donors, the probability of a window-period donation, the probability of lab error and the possibility that a donation would be discarded because of the results of screening tests for other diseases, such as hepatitis.