The Food and Drug Administration (FDA) approved Chiron Corporation's sophisticated genetic fingerprinting tests yesterday for blood banks to use to reduce the risks of the AIDS or hepatitis C viruses slipping into transfusions. The vast majority of transfusions are already infection-free. Blood banks have been performing the nucleic acid test (NAT) as part of a nationwide experiment since 1999 to see if the more sophisticated method can make the blood supply even safer.

In the three-year study, the new test caught 7 blood donations with HIV and 88 with 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 the viruses 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 may not occur for weeks, even months after an infection. NAT closes much of that window period, cutting off about a week for HIV and 57 days for hepatitis C. In practical terms, that means NAT can prevent up to 50 cases of hepatitis every year, and cut the HIV risk to 1 in 2 million transfusions.

The cost of NAT will reach $15 to $20 a donation, up from the lower research cost of $8 a donation, said Jim McPherson, of America's Blood Centers. Chiron Corp. sells the test under the name Procleix. Roche Diagnostics is seeking approval of a slightly different version of NAT, which remains in testing in a number of blood banks. It will take the FDA about a year to order blood banks to use the test, time in which either Roche's competing test will be approved or blood banks will have to switch to Chiron's.

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