A modified absorption-reduction method to detect virus-specific hemagglutination inhibiting and neutralizing IgM antibodies.

IgG antibodies are preferentially absorbed by protein A-coupled sepharose beads while most of the IgM and IgA antibodies remain in solution. Even relatively low amounts of virus-specific IgM antibodies can then be detected unequivocally by a decrease of the antibody titer following the treatment with reducing agents. Ethandithiol proved superior to 2-mercaptoethanol in combination with neutralization assays.