Q fever is often diagnosed based on serologic testing results of a patient with suggestive clinical findings. Testing involves measuring titers of IgG and IgM antibodies against Phase I and Phase II antigens. These correspond to smooth and rough lipopolysaccharides formed by intracellular organisms.
NOTE: Many of the references report data as inverse titer (i.e., a value of X indicates a titer of 1/X.)
Assays:
(1) microimmunofluorescence (most widely used)
(2) complement fixation
(3) ELISA
Caveats:
(1) Late seroconversion can result in false negative diagnosis if the samples are taken too early in the course of the disease.
(2) When following titers samples should be taken 2 or more weeks apart.
Diagnosis
Antibody Titers by Microimmunofluorescence
acute Q fever
inverse titer anti-Phase II IgG >= 200 AND > anti-Phase I IgG, PLUS
inverse titer anti-Phase II IgM titer >= 50 AND > anti-Phase I IgM
chronic Q fever
inverse titer anti-Phase I IgG >= 800 AND >= anti-Phase II IgG
Q fever excluded
inverse titer anti-Phase II IgG <= 100 collected >= 1.5 months after onset of symptoms
where:
• Dupont et al demonstrated that testing for IgA antibodies did not help in making the diagnosis (Fournier et al, J Clin Micro, 1998, page 1830).
Diagnosis
Complement Fixation Antibody Titers
acute Q fever
inverse titer anti-Phase II >= 40
chronic Q fever
inverse titer anti-Phase I >= 200
ELISA - Q fever is present if one of the following is detected:
(1) seroconversion
(2) 4 fold or greater increase in antibody levels
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