Q fever endocarditis is a lifethreatening disease for which the diagnosis is usually based on serology. The major microbiologic criterion for the diagnosis of infectious endocarditis (two separate positive blood cultures) cannot be achieved in most routine laboratories because of the biohazard associated with the culture of Coxiella burnetii, the etiological agent of Q fever.

Recently, new criteria for the diagnosis of infectious endocarditis have been proposed, and in this study we attempted to assess the suitability of these criteria specifically for the diagnosis of Q fever endocarditis.

To achieve this aim, we first selected from our series 20 recent cases in whom endocarditis had been confirmed following valvular pathological examination, and for whom microbiological evidence for the involvement of C burnetii was available. Then, we applied the criteria proposed by the Duke Endocarditis Service (ie, C burnetii positive serology being considered a minor criterion) to this cohort of patients but excluding pathological findings. Although the Duke Endocarditis Service criteria confirmed diagnosis in 16 of the patients, 4 were misclassified as “possible” cases (20%). However, when the Q fever serological results (using an 1/800 antiphase I immunoglobulin G cut off) and single blood culture results were changed from minor to major diagnostic criteria, endocarditis was confirmed in them all. A second time, prospectively, we applied the Duke Endocarditis Service criteria to a further 5 patients affected with Q fever endocarditis. Strict application of these criteria resulted in 1 of the 5 being misdiagnosed. Applying the suggested modification for C burnetii results, all 5 were confirmed as having infectious endocarditis.

We propose that the modifications discussed in this study be applied to the Duke Endocarditis Service criteria in order that the diagnosis of C burnetii induced endocarditis is improved.