The specific antibody coated turkey cell system (Hepatest) has established itself as a reliable, inexpensive, convenient and sensitive test for the detection of Hepatitis B antigen in serum. The test employs a one % suspensionof turkey erythrocyte coated with antibody to hepatitis โฌ3 surface antigen. The antibody is purified by immunoadsorption columns to increase specificity. The coated cells are therefore agglutinated in the presence of hepatitis B antigen. False positive reactions can be caused by other substances, eg, antibodies to turkey red cells which may be present in some sera. Such antibodies can be removed by incorporating normal turkey serum which contains antigens in the buffer used to suspend the cells. In order to further reduce the incidence of false positives the first dilution of serum is usually 1/8.
In a titration of a standard serum, for example, solid-phase radioimmunoassay ("AusRIA 11,'' Abbott Laboratories) would give a titer of 1 /lOO,OOO and the reversed passive hemagglutination test ("Hepatest") 1/20,000. In effect, the sensitivity of Hepatest for screening is reduced another eight-fold due to the initial screening dilution. The practical importance of the greater sensitivity is slight, only a few percent more positives being identified as most positive samples contain large quantities of antigen.