Flow diagram showing the evolution of Q fever disease in the absence of treatment. Q fever disease starts with asymptomatic primary infection (0-10 days), followed by acute Q fever (10 days to 3 months), and some subjects then develop the chronic form of disease (>3 months). The clinical sample used initially for the detection of C. burnetii at each stage is patient serum. The strategy for early-stage Q fever diagnosis consists of combined approaches, including PCR (≤7 days) and antigen detection by immunofluorescence assay (IFA) or enzyme-linked immunosorbent assay (ELISA) (≥7 days) performed on whole-cell antigen (formalin-inactivated bacteria). Immuno-PCR (IPCR) performed with whole-cell protein extracts may also be a promising detection tool. The diagnosis of chronic Q fever relies mainly on serology. The cut-off stands at (i) IgM phase II ≥25 and IgG phase II (and I) ≥200 for acute Q fever serodiagnosis; and (ii) IgG phase II and I ≥1,600 associated with the presence of IgA phase I ≥50 for chronic Q fever serodiagnosis. IgM may be still detectable in cases of chronic Q fever. The protein candidates for serodiagnosis selected by several proteomic studies are shown in the circles. In the centre, the most antigenic proteins, namely CBU_1910 (Com1) and CBU_1718 (GroEL), as well as whole-cell antigen, are versatile markers of Q fever.