Blood banks do all they can to ensure that donations carry no pathogens that could infect and

possibly kill recipients. But screening tests for the microorganisms that cause some tropical diseases, such as dengue and chikungunya, do not exist, and these pathogens have been spreading into the U.S. in recent years because of global warming. Meanwhile tests for viruses such as HIV and hepatitis C can take up valuable time, and pathogens that have not yet been identified may be lurking in blood, as happened in the early days of HIV.

Now U.S. blood banks have a way to clear donations of pathogens: last December the Food and Drug Administration approved the INTERCEPT Blood System, making it the first technology available to rid platelets (the clotting components of blood) and plasma (the fluid) of nearly all possible infectious agents. Developed by Cerus, the technology mangles the nucleic acids (RNA or DNA) in viruses and bacteria, thereby preventing the pathogens from reproducing in a recipient's body. Technicians first add a molecule capable of inserting itself into the DNA or RNA to the donated material, then expose the mixture to ultraviolet light (right). The light causes the molecules to bind irreversibly to the nucleic acids and thus prevent their replication. The procedure does no harm to the plasma or platelets because they contain no nucleic acids of their own. The procedure varies slightly for red blood cells (which also lack nucleic acids)—a use that the fda has yet to approve. Before this technique became available in the U.S., blood donations from chikungunya- and dengue-afflicted areas had to sit on the shelf for two days while donors were monitored for disease symptoms—a difficult constraint because platelets have only a five-day shelf life.

Europe has relied on the INTERCEPT system since 2002, but the fda withheld its approval until postmarket data on safety and efficacy became available—and the threat of dengue and chikungunya in the U.S. grew. This summer SunCoast Blood Bank in Florida and Blood Bank of Delmarva, which serves parts of Delaware, Maryland and Virginia, are the first U.S. blood banks to use the technology. The National Institutes of Health also signed a supply agreement with Cerus in May, and a recent New England Journal of Medicine editorial advocated for a national mandate to use a system such as INTERCEPT to reduce risks from pathogens.

“We in the U.S. probably have the safest blood supply in the world,” says SunCoast CEO Scott Bush, “but this technology offers an extra layer of protection.”