February 28, 2002
FDA also recently licensed the first NAT system for screening donors of plasma for the specific use in products that will be further manufactured, such as clotting factors and immune globulins.
The approved test system was developed by Gen-Probe Inc., San Diego, Calif. and will be distributed by Chiron Corporation, Emeryville, Calif.
Blood donors have been tested for evidence of HIV infection since 1985 and for evidence of HCV infection since 1990. Although increasingly sensitive tests for detection of HIV and HCV antibodies and HIV antigen were implemented during the past decade, in rare instances infections in donors have been missed.
The NAT system is capable of detecting more infectious donations than current tests because it detects viral genes rather than antibodies or antigens (proteins from the virus). Detection of viral genes permits detection earlier in the infection since the appearance of antibodies requires time for the donor to develop an immune response, and since detection of antigens requires time for a higher level of virus to appear in the bloodstream.
This new technology detects very small amounts of genetic material by copying the genes numerous times, resulting in a billion-fold amplification of the target gene. The approved test system can detect ribonucleic acid (RNA) from HIV-1 and HCV when tested in pools of 16 samples obtained from multiple donors. In a less automated format, it can also be used to test individual samples from whole blood collections. If a test pool is positive for either virus, the individual donation suspected of containing a virus can be identified and not transfused. The donor can be deferred from donating blood and notified.
Currently, donors of blood and plasma are tested for antibodies to HCV, antibodies to HIV and HIV-1 antigens, which are the virus's own proteins. However, there is still a "window period" during which a donor can be infected, but have negative screening tests. With the use of NAT for HCV, the window period is reduced by approximately 57 days (from an average of 82 days to 25 days). For HIV-1, the average window period with antibody is 22 days. This window period is reduced approximately to 16 days with antigen testing and to 12 days with NAT.
In nation-wide clinical trials performed to support the approval of the test on pools, a total of 7 HIV-1 positive and 88 HCV positive donations were detected in more than 20 million donations tested confirming the effectiveness of the test. The NAT system using pools was evaluated at eight volunteer blood donor sites while NAT for use with individual donations used data from U.S. military blood donor sites.
The use of the licensed test will allow blood banks that implement it to discontinue antigen testing, although blood donations will continue to be tested by antibody tests. FDA plans to issue guidance on the use of NAT in the near future.
Since 1997, FDA has encouraged the large-scale study of NAT through the use of experimental protocols. More than one test system is under development. Most of the nation's blood establishments now participate in these experimental protocols. The Gen-Probe NAT system is the first to be approved to screen donors of whole blood and blood components intended for use in transfusion.