WASHINGTON -- Several hundred transfusion recipients around the country -- adults undergoing heart surgery and children with certain inherited anemias -- are being enrolled in a bold experiment: They'll receive donated blood that has essentially been sterilized.

Today's blood supply is safe because it undergoes numerous tests to ensure that pints containing infections like the AIDS virus are thrown out.

Testing isn't perfect, however, and a few tainted pints slip by. Last month's discovery that West Nile virus apparently can be spread through transfusions highlights another problem: Blood banks can't prevent what they can't test for.

A technology called "path-ogen inactivation" is being promoted as the solution. A chemical added to a donated bag of blood is supposed to break up the genetic material of any viruses or bacteria lurking inside so they can't infect, essentially offering the first way to sterilize blood.

"This is a paradigm shift in the way we deal with protecting the safety of the blood supply," says Dr. Laurence Corash of Cerus Corp., one of several companies developing the technology. "In the past we have relied upon testing. Testing will never put you in front of a new pathogen like West Nile virus."

Now Cerus and a competitor, V.I. Technologies, are beginning the first major U.S. studies, in 520 patients, to see if it's safe to give people pathogen-inactivated red blood cells.

Cerus has won European approval to sterilize platelets, one component of blood, and marketing should begin by the end of the year.

But don't expect sterilization of red blood cells -- the typical transfusion -- for at least several more years, as researchers study some serious questions: Does the cleaning alter blood so it doesn't function normally? Does the leftover cleanser residue pose any risk?

'Somewhat in the future'

The Food and Drug Administration calls pathogen inactivation highly promising. It even took the unusual step of asking Cerus and Vitex to turn over evidence that their technologies could destroy West Nile virus in transfusions, raising the prospect of additional clinical trials if the mosquito-borne virus renews its threat next summer before a blood test, expected by then, is working.

"The promise is somewhat in the future," cautioned FDA's Dr. Jesse Goodman. The FDA will demand stringent proof that altering blood to eliminate a bug found in, say, one of every 5,000 donated pints won't pose a new and different risk to the other 4,999.

Red blood cells, plasma and platelets, blood's major components, contain no nuclear DNA or RNA. White blood cells have genetic material, but as they're not needed in transfusions, destroying them does no harm.