The HI titre is also influenced by the characteristics of
antigen used. For instance, the use of the homologous La Sota antigen in the HI assay after vaccination with
this virus resulted in significantly higher titres than when heterologous Ulster virus was used (Maas et al.,
1998). Furthermore, reference antigens produced with historic strains may reduce the sensitivity of HI assay
when used for the detection of antibodies against ND viruses currently circulating. For this reason it is
important to investigate the antigenic relationships between the antigen used in the laboratory and current
circulating viruses, and between vaccine strains and reference HA antigens, to avoid misjudgements in
estimating serum antibody titres.