Why were samples weakly positive for IgG antibodies not tested for IgA antibodies?

EDITOR—The relation of Chlamydia pneumoniae antibodies to ischaemic heart disease reported by Strachan et al, and the commentary on the paper, deserve comment.1 An association is reported between IgA antibody at the start of the study and prevalent ischaemic heart disease or subsequent death due to ischaemic heart disease. Presumably, serum samples with an IgG antibody titre of <1 in 16 were not tested for IgA antibody because it was thought that IgA antibody would not exist. Why, however, were 310 samples that were weakly positive for IgG antibodies not tested for IgA antibodies? Surely some of these could have been weakly positive for IgA antibodies at 1 in 16 and so have influenced the interpretation of results?

Persistent IgA antibody is thought to indicate a chronic infection, as is persistent IgG antibody. Conceptually, therefore, it is difficult to understand why significance should apply apparently only to the presence of IgA antibodies, particularly as others have shown an association with IgG antibodies. Furthermore, as West mentions, there is a problem of relating events to antibody measured only once.1 Obviously, IgA antibodies may persist but also disappear. We wonder what …