Diagnosis is based on PCR or serology.

PCR can be performed on whole blood or serum in the acute phase, and allows the diagnosis of acute Q fever within the first 2 weeks of infection. PCR can also be performed on infected tissues (heart valve samples, liver biopsies, bone biopsies, …).

Serology can be performed using a highly sensitive screening test followed by an immunofluorescence test for confirmation which also allows serum titration.
A fourfold increase in phase II IgG antibody titer by immunofluorescence between paired acute and convalescent specimens is the gold standard for confirming the diagnosis of acute Q fever. However, negative serology in the acute phase does not exclude Q fever.  Indeed, immunofluorescence is negative during the early stages of acute disease, while PCR is usually still positive. Most patients seroconvert before the third week of illness. A single high “convalescent” serum collected after the acute phase of the disease is sufficient for diagnosis (positive if >1/256); however, a quadruplicate between acute and convalescent specimens has much higher sensitivity (can be considered positive with titers >1/64) and specificity than a single high convalescent titer.
Diagnosis of chronic Q fever requires demonstration of a high phase I IgG antibody titer (≥ 1:800) greater than the phase II IgG titer and an identifiable focal infection (e.g., endocarditis, hepatitis, or spondylodiskitis).

PCR, immunohistochemistry, or culture of affected tissue can provide definitive confirmation of C. burnetii infection. However, these approaches have low sensitivity and are somewhat difficult to implement because specimens are not routinely available. Thus, the diagnosis of Q fever relies heavily on serologic monitoring of the patient. It should be noted that immunofluorescence requires considerable expertise. Specific and relatively sensitive PCR is also a challenge and few laboratories offer it.

All the methods mentioned for detection by molecular biology and serology are available at the CNRT.