Chlamydia pneumoniae Antibodies, IgM

Chlamydia pneumoniae Antibodies, IgM (also reported as Chlamydophila pneumoniae IgM) is used to assist in diagnosing upper respiratory infections and atypical community‑acquired pneumonia attributable to C. pneumoniae, and to help separate atypical causes of pneumonia from other etiologies. Demonstration of IgM reactivity and/or a rising titer in paired acute and convalescent sera can retrospectively confirm a recent infection. Utility is limited for reinfections, in which an IgM response may be minimal or absent. Whenever available, results are weighed together with molecular detection methods.

Chlamydia (Chlamydophila) pneumoniae is an obligate intracellular bacterium associated with upper respiratory tract disease—including laryngitis, pharyngitis, sinusitis, otitis media, and bronchitis—as well as atypical pneumonia. Illness is seen across ages but is frequent among children, adolescents, and older adults. Transmission is person to person, and clusters may occur in closed or crowded environments. Asymptomatic carriage and persistence in the respiratory tract are common. Because culture is technically challenging, laboratory diagnosis relies on serologic evidence of a change in antibody levels between paired specimens—one collected during the acute phase and a second approximately two weeks later—and/or detection of bacterial DNA by PCR. IgM antibodies typically appear earliest after a primary infection and remain measurable for about 2–3 months. In reinfection, the IgM response may be absent or only slightly elevated. Concordant positive serologic and PCR results increase the likelihood of active infection.