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Background: Acute and chronic Q fever/Coxiella burnetii infection is diagnosed principally by serology. The management of patients who have serological evidence of chronic Q fever but no other manifestation of chronic infection is challenging., Methods: This paper describes a follow-up study of individuals 6 years after a point source outbreak. The study compares serological and polymerase chain reaction (PCR) results between 3 international reference laboratories in a well-defined cohort of Q fever patients., Results: Concordance in microimmunofluorescence result interpretation from the 3 centers was only 35%. Australian and UK results had the greatest concordance and French and UK results the lowest. Serological testing revealed no chronic serological profiles when tested in either France or Australia but 10 when tested in the UK. Serological results from a patient with treated Q fever endocarditis suggested treated (France), chronic (UK), and borderline chronic (Australia) infection. PCR results on blood were universally negative., Conclusions: This study has shown that the results from Q fever micro-immunofluorescence vary according to the center in which they are carried out. This has implications for the interpretation of such tests, raises questions regarding the validity of using serological criteria alone as a means of diagnosing chronic Q fever, and affects the interpretation of epidemiological studies. We recommend that all results are interpreted according to the clinical picture and particular caution is applied in the interpretation of chronic serological profiles. In order to further our understanding of Q fever infection we propose that an international standard of Q fever serological investigation be developed.