The validity of the criteria for primary infection of Chlamydophila pneumoniae in children by measuring ELISA IgM antibodies

As IgM antibody measurement by enzyme-linked immunosorbent assay (ELISA) has become possible for the serological diagnosis of Chlamydophila pneumoniae (C. pn) infection, the HITAZYME-ELISA method has become widely employed in Japan. However, in children, when the diagnostic criterion of primary infection is set at ID ≥1.1, the positive rate is higher than expected, and the potential for inaccurate reflection of the prevalence has been raised. In this study, we performed ROC analysis involving 136 pediatric patients with acute airway symptoms (0–14 years of age), considering a 32-fold or higher micro-immunofluorescence IgM antibody titer against C. pn as positive. Setting the cut-off value for ELISA C. pn IgM antibody ID at 2.0, the specificity was 100%, with no false positivity. The maximum (sensitivity + specificity)/2 was obtained when the cut-off value was set at 1.5. Therefore, IgM ID ≥2.0 was regarded as definitely positive and an IgM ID between 1.5 and 2.0 was regarded as indeterminate as diagnostic criteria for the primary infection. When the prevalence was investigated in 3,108 children (0–15 years of age) with airway symptoms based on these criteria, 542 cases (17.4%) were positive, and the median duration of IgM antibody positivity was five months. Long-term positivity (ten cases) for more than 12 months and recurrent positivity (eight cases) were also observed, but it may be appropriate to set a new criterion of IgM antibody ID ≥2.0 for the diagnosis of primary Chlamydophila pneumoniae infection in children.