Blood collected by 11 Red Cross Regional Blood Centers were screened by the method of agar gel diffusion (AGD) for the presence of hepatitis-B antibody. Of the 185,134 units tested, 114 were found to be positive for HBAb by the Regional Centers and were forwarded to National Special Projects Laboratory for confirmation. Only five out of 114 samples revealed lines of identity with a control anti-HBAg when reacted with a pool of plasma positive for hepatitis-B antigen that was different from that which was provided to the Centers. Apparently, the precipitation reactions observed by the Centers were largely due to the antithrombin antibodies in the donors' sera reacting with the residual thrombin used by the manufacturer to convert HBAg positive plasma to serum. We conclude that the incidence of hepatitis-B antibody as measured by agar gel diffusion in the Red Cross Blood donor population was extremely low.
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