Assay of togavirus haemagglutination-inhibition antibodies by the micro method: loss of information and its rectification.

Haemagglutination inhibition (HI) activity was assayed by the micro and macro methods in immune sera prepared against four togaviruses. HI titres were always 4 to 8 times lower by the micro method, and coincided with 4 to 8 times lower haemagglutinin titres in micro method assays. Because of this phenomenon, positive sera with HI macro method titres lower than 1: 80 will be false negative for HI by the micro method when tests begin at a 1: 10 serum dilution. This was confirmed for a number of human sera tested for West Nile virus HI activity. As a consequence of the difference between titres given by the two assay methods, results of seroepidemiological studies may be distorted. Use of a system that combines both the macro and micro methods could rectify this distortion.