Filed at 1:21 p.m. ET

WASHINGTON (AP) -- The government approved sophisticated genetic fingerprinting tests Thursday for blood banks to use to reduce the risk of the AIDS or hepatitis C viruses slipping into transfusions.

The vast majority of transfusions are infection-free. But blood banks have been performing this nucleic acid testing, called NAT, as part of a nationwide experiment since 1999 to see if the more sophisticated method can make the blood supply even safer.

Thursday's approval by the Food and Drug Administration validates NAT testing as adding that extra layer of safety.

In the three-year study, the new test caught seven blood donations tainted with HIV and 88 tainted with liver-destroying hepatitis C that otherwise would have slipped into the blood supply, the FDA said.

``Blood is already remarkably safe,'' said FDA blood chief Dr. Jay Epstein. ``However, the public wants the safest achievable blood supply and these are technological innovations which do make blood safer.''

NAT can detect tiny amounts of HIV or hepatitis C before the blood donor's body has even recognized the infection. That's because viral genes spread through blood faster than the immune system begins forming antibodies to fight them, a reaction that may not occur for weeks, even months after the person is infected.

Yet until now, blood testing has depended largely on tests that detect antibodies in an infected donor. So newly infected donors occasionally slip through.

Nucleic acid testing closes much of that ``window period,'' cutting off about a week for HIV and a stunning 57 days for hepatitis C. In practical terms, that means NAT can prevent up to 50 cases of hepatitis C every year, and cut the HIV risk to 1 in 2 million transfusions, Epstein said.

``We're very excited,'' said Jim McPherson of America's Blood Centers, whose member blood banks collect half of the nation's supply. ``This exceedingly powerful tool is now recognized as improving blood safety.''

``As we continued to do it and identify donors who were positive on NAT but not detectable on other tests, it became clearer and clearer'' the new testing was valuable, added Dr. Jerry Squires of the American Red Cross, which supplies the other half of the nation's blood. ``This is, we think, a significant step.''

But McPherson said the FDA's approval means the cost of NAT will reach $15 or $20 a donation, up from the lower research cost of $8 a donation.

That's partly because NAT is an intensive process to perform, requiring a specially vented and cleaned laboratory to prevent contamination, extra preparation of blood samples and more time. The test materials will also cost more.

Chiron Corp. makes the newly approved test, which it developed with Gen-Probe Inc. and sells under the brand name Procleix.

A competitor, Roche Diagnostics, is seeking FDA approval of a slightly different version of NAT, which remains in testing at a number of blood banks. It will take the FDA about a year to order all blood banks to begin using a licensed version of NAT instead of the research version, time in which either Roche's competing test could be approved or banks would have to switch to Chiron's.

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On the Net:

Food and Drug Administration: http://www.fda.gov

America's Blood Centers: http://www.americasblood.org

American Red Cross: http://www.redcross.org