The limitations of IgM assays in the serological diagnosis of Mycoplasma pneumoniae infections.

The most useful and reliable serological investigations for the diagnosis of current Mycoplasma pneumoniae infection, including reinfection, were investigated. Paired sera and respiratory specimens from 115 patients with lower respiratory tract symptoms were examined for evidence of current M. pneumoniae infection by serological response, as measured by complement-fixation and indirect immunofluorescence tests for specific IgM, IgA and IgG, and also by culture of M. pneumoniae from respiratory material. Specific IgM was not always detectable in cases where other criteria indicated current or recent infection. On the basis of the present results, it is postulated that primary infection and reinfection may be differentiated by the presence or absence of specific IgM in the presence of elevated specific IgA levels and, therefore, that estimation of both IgM and IgA is necessary for the maximal detection of current M. pneumoniae infection, including reinfections. Specific IgG levels remained elevated for many weeks and were not useful diagnostically.