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J Infect Chemother. 2013 Apr;19(2):256-60. doi: 10.1007/s10156-012-0494-4. Epub 2012 Oct 13.

Chlamydophila pneumoniae serology: cross-reaction with Mycoplasma pneumoniae infection.

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Department of Internal Medicine I, Kawasaki Medical School, 2-1-80 Nakasange, Kita-ku, Okayama, Japan.


Atypical pathogens Mycoplasma pneumoniae and Chlamydophila pneumoniae play an important role in community-acquired pneumonia. However, it has been pointed out that positive enzyme-linked immunosorbent assay (ELISA, Hitazyme C. pneumoniae) IgM reactivity is frequent among M. pneumoniae pneumonia patients. To clarify the reactivity of ELISA IgM in M. pneumoniae pneumonia, findings were compared with immunoblotting, ELNAS Plate C. pneumoniae (ELNAS) and the micro-immunofluorescence (MIF) test. Ninety-eight serologically confirmed cases with M. pneumoniae pneumonia and 10 cases with C. pneumoniae pneumonia were enrolled in this study. C. pneumoniae IgM-positive cases measured by the ELISA were observed in 30 (30 %) patients with M. pneumoniae pneumonia. However, there were no positive cases by immunoblotting, ELNAS, or MIF test. These cases determined to be IgM positive only in the ELISA were all negative by another serological test, recombinant enzyme immunoassay (rEIA), and these positive results in the ELISA were considered to be false-positive reactions. In contrast, IgM-positive findings in patients with C. pneumoniae pneumonia did not show any positive reaction in M. pneumoniae antibody titer. ELISA showed a high frequency of false-positive findings in patients with M. pneumoniae pneumonia, which included false-positive cases with a high titer for IgM. To accurately diagnose C. pneumoniae infection in various studies, including respiratory infections, researchers should consider the IgM false-positive reaction with ELISA in patients with suspected atypical pneumonia.

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