Correlation between serum doxycycline concentrations and serologic evolution in patients with Coxiella burnetii endocarditis

The recommended treatment for Q fever endocarditis is a combination of doxycycline and hydroxychloroquine. We found a correlation between serum doxycycline concentrations and decreases in levels of phase 1 Coxiella burnetii antibodies, in 24 patients with Q fever endocarditis. Patients who had a >2-fold decrease in levels of phase 1 antibodies had serum doxycycline concentrations higher than those of the other patients (mean +/- SD 5.29 +/- 1.75 3.14 +/- 1.40 mug/mL; P = .003). We recommend adjusting the posology of doxycycline to achieve a serum concentration of at least 5 mug/mL.