Q fever laboratory tests

Laboratory findings consistent with the diagnosis of Q fever include positive serology for antibodies (especially indirect immunofluorescence (IIF), positive PCR, and elevated liver enzymes.

• Indirect immunofluorescence (IIF) is the method of choice for antibody detection and is preferred over ELISA and complement fixation.^[1][2]
• Antibodies start to be detected after 7-14 days of infection, with most patients testing positive by the third week.
• Anti phase II antibodies are tested first. If positive, anti phase I antibodies are tested.
• After acute infection, serologic follow-up for serum anti phase I IgG antibodies. The test is done twice every 3 months for 2 years. If it’s positive, a transesophageal echo should be done to rule out endocarditis.^[3]
• All serologic test results should be used in the context of clinical data because false positive test results are seen in many other diseases (e.g. leptospirosis).

• PCR can be used to detect C. burnetii DNA in cultures and clinical samples.
• PCR is positive in the first week of infection, thus it can be used to diagnose Q fever in patients who are serologically negative in the early stages of the disease.^[4]
• Quantitative PCR also can be used in patients whose anti phase II IgG antibodies are persistently positive in order to detect chronic Q fever.

• C. burnetii doesn’t grow on ordinary blood cultures but can be cultivated on special media as embryonated eggs or cell culture.
• C. burnetii is extremely infectious and samples should be handled with caution.

• A two-to-three fold increase in AST and ALT is seen in most patients.