Screening for antibodies against infectious bronchitis virus: specificity of the haemagglutination inhibition test

A screening of antibodies against strain M-41 and the Dutch variant strains D-274 and D-1466 of infectious bronchitis virus (IBV) using the haemagglutination inhibition test (HI) was carried out in Chile. The presence of these variant strains has not been reported yet and therefore the probability of finding positive flocks is low. On this basis some statistical analysis were made with those data obtained in order to contribute to the problem of the specificity of the HI test for IBV diagnosis. All sera were distributed in different categories of possible results against each strain according with the HI titre obtained and employing different discrimination levels for a positive or negative status against IBV. It was concluded that discrimination levels varying from 4.0 to 5.0 log2 are useful for establishing the presence or absence of antibodies against IBV but if a serotype specific diagnosis is needed, then the discrimination level should be elevated, specially when secondary immune responses are measured.