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Two types of tests can assist the diagnosis:

  1. Immunological antibody response can be assessed using serology assays. Different laboratories may use immunofluorescence (IF) assays, enzyme immunoassay (EIA) or complement fixation (CF) based tests or combinations of these to detect different classes of antibodies to phases II and I of Q fever bacteria. IF and CF tests may be reported as titres proportional to the amount of antibody in a patient’s blood. Significant titres may take 3-4 weeks to appear so blood samples should be taken initially during acute illness and then at least four weeks after the onset of the disease.
    Paradoxically, antibodies to the phase II organism are high in acute disease, and antibodies to the phase I organism are raised in chronic disease. In acute Q fever phase II IgM and IgG antibodies are detected. For chronic Q-fever development of a phase I IgG IF titre >800 and possibly exceeding the phase I IgG IF titre is suggestive of chronic Q-fever. However, elevated phase I IgG titres may persist for a several months after acute infection without the disease progressing to chronic disease.
    Q fever serology results are often complex and should be interpreted by a clinical microbiologist or infectious disease specialist.
  2. The actual pathogen can be detected by molecular testing using the polymerase chain reaction (PCR). It holds the promise of timely diagnosis, since it should be positive before antibodies are detectable. However, the average sensitivity of PCR performed on blood has been low (~20%). PCR is much more sensitive on tissue samples such as heart valves, which accumulate much higher concentrations of bacteria than serum.
    Laboratory cultures of Q fever bacteria are very complex and dangerous and are not used for diagnostic purposes.

Last Review Date: December 4, 2020