Coxiella Burnetii ELISA

Results should always be interpreted in the context of the patient’s clinical presentation, epidemiological exposure, immune status, stage of infection and any other relevant diagnostic findings. The diagnosis of Coxiella burnetii infection should never be based on a single laboratory result.

Serology may be negative during the early stages of acute infection, as antibodies are not always detectable during the first 1–2 weeks of illness. If the first sample is collected early and the result is negative, a second sample should be obtained 14–21 days later and tested in parallel with the first sample to assess possible seroconversion. IgM results should not be interpreted in isolation, as these antibodies may persist for months after infection and may occasionally show non-specific reactivity. In general, IgG-based interpretation is more informative for the diagnosis and follow-up of Q fever, especially when phase-specific serological patterns are available.

A single positive antibody result may reflect recent infection, past exposure, background seropositivity or non-specific reactivity. In cases of suspected chronic Q fever or persistent focal infection, high Phase I IgG titres may support the diagnostic suspicion, although serology alone is not sufficient to establish this diagnosis. Chronic Q fever requires compatible clinical findings together with evidence of a persistent focus, such as endocarditis, vascular infection or other focal manifestations. In these contexts, IgM is usually not detectable.

PCR is especially useful during the early phase of acute infection, before seroconversion. However, a negative PCR result does not exclude Q fever, especially after seroconversion or following antibiotic treatment. Serological cross-reactions may also occur with other microorganisms, such as Legionella, Bartonella, Ehrlichia and Rickettsia species; therefore, results should always be interpreted with caution in the appropriate clinical context.

Special attention is required in certain patient groups. In immunocompromised individuals, the antibody response may be weak, delayed or atypical, and negative serology does not always exclude infection. In neonates, IgG results should be interpreted with caution, as maternal IgG may have been passively transferred before birth. Finally, the reliability of results also depends on appropriate sample collection, transport, storage and processing.