Ryan White, suffering from the blood disorder hemophilia, was just 13 in 1984 when he received a blood transfusion tainted with the AIDS virus. He became a symbol of the epidemic as it crossed demographic lines, but his case also brought to light the need for methods to test donated blood for HIV and other diseases that could be transmitted to unsuspecting recipients. Back then, up to 1 percent of all blood units in the United States contained either HIV or hepatitis C. New screening techniques have been introduced since then, and two separate studies from the American Red Cross update the safety statistics on the blood supply and donated human tissue

What the researchers wanted to know: How safe is the U.S. blood supply from viral infections and what is the rate of infection from donated human tissues?

What they did: In the first study, on blood supplies, the researchers compiled data from more than 40 million blood donations and analyzed whether a recently developed and almost universally used test, known as a nucleic acid-amplification test, was better at detecting HIV and hepatitis C than older tests. In the second study, to assess the safety of tissues such as heart valves, veins, or bones (but not organs), the researchers used data from tissue donation centers from around the country to estimate the rate of viral infection in tissue donors. They analyzed 11,391 tissue samples that were tested for HIV, hepatitis C, hepatitis B, and human T-lymphotropic virus, which causes a disease of the immune system, at centers around the country. Because tissue samples are tested using a technique that may not capture viral infection in its earlier, latent stages, the researchers also used demographic data to estimate the rate of disease among donors unaccounted for in reports from tissue donation centers. Since tissue donors are almost always dead, they can't be asked all the questions that blood donors have to answer about their potential for exposure to different diseases.

What they found: The blood supply has become much safer in the past 20 years; with new testing techniques, the risk of infection with either HIV or hepatitis C is 1 for every 2 million units. The authors estimate that nucleic acid-amplification screening catches about five cases of HIV-infected blood that would not otherwise be caught and 56 cases of hepatitis C that would be transfused into someone else each year. The second study, on infection from tissue transplants, is not as encouraging. Tissue donors are less likely than the general population to be infected with the viral infections but more likely to be infected than blood donors. The researchers estimated that the probability that a tissue will be infected but undetected is 1 out of 55,ooo donations for HIV, 1 out of 34,000 for hepatitis B, 1 out of 42,000 for hepatitis C, and 1 out of 128,000 for human T-lymphotropic virus. In other words, at the highest, one donor per year has a viral infection that passes through the system without detection. However, because tissue products can be used on multiple patients, the average is 50, one donor could affect a good number of people.

What it means to you: The odds of receiving blood tainted with hepatitis C or HIV are now about the same as the odds of being killed by a large meteor impact (at least according to NASA's calculations)—so it's probably not something you should lose sleep over. Similarly, the odds of receiving infected tissue, while greater, are still quite small. The authors say that the same test that has improved blood safety should also be used for human tissues but acknowledge it would cost millions of additional dollars for the chance of eliminating one infected sample per year.

Caveats: These risks only pertain to those diseases listed above; some rare viruses can still slip through the screeners. Some other viruses, such as West Nile, are being kept out of the blood supply, but screening is relatively new and more research needs to be done to determine the reduction in risk from these programs.

Find out more: The American Association of Blood Banks has a very long Web page with all you've ever wanted to know about blood donation and more.

The American Red Cross also has information about donating blood and tissue, linked from their home page.

Read the articles: Stramer, S.L. et al. "Detection of HIV-1 and HCV Infection Among Antibody-Negative Blood Donors by Nucleic Acid-Amplification Testing." New England Journal of Medicine. Aug. 19, 2004. Vol. 351, No. 8, pp. 760–768.

Abstract online: http://content.nejm.org

Zou, S. et al. "Probability of Viremia With HBV, HCV, HIV, and HTLV Among Tissue Donors in the United States." New England Journal of Medicine. Aug. 19, 2004. Vol. 351, No. 8, pp. 751–759

Abstract online: http://content.nejm.org