Chlamydia pneumoniae Antibody, IgG

Chlamydia pneumoniae Antibody, IgG (also reported as Anti‑Chlamydia pneumoniae IgG or Chlamydophila pneumoniae IgG) is used to support evaluation of respiratory infections attributed to C. pneumoniae and to assist in the differential diagnosis of community‑acquired pneumonia with atypical features. It is especially useful for retrospective confirmation by documenting seroconversion or a fourfold increase in IgG between acute and convalescent specimens. Results are interpreted alongside clinical findings and, when available, nucleic acid amplification testing (PCR), given that serology alone cannot reliably establish timing of infection from a single specimen.

Chlamydia (Chlamydophila) pneumoniae is an obligate intracellular bacterium that infects the human respiratory tract. Person‑to‑person transmission leads to outbreaks in settings with close contact, and asymptomatic persistence in the airways is common. Conventional culture is difficult; therefore, diagnostic strategies rely on serology and PCR interpreted in the context of clinical presentation. For acute infection, a fourfold increase in antibody signal between properly timed acute and convalescent sera confirms recent C. pneumoniae infection. IgG typically emerges several weeks after infection and may remain detectable for up to two years; an elevated IgG titer without a significant change on repeat testing most often reflects prior infection or colonization/persistence rather than new disease. Clinically, C. pneumoniae pneumonia often follows an atypical course with gradual onset, frequently beginning with upper respiratory tract symptoms and a dry cough. Leukocytosis may be absent, and chest imaging can show small segmental infiltrates. Severe illness can occur in older adults.