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 vs. 3.14+/-1.40 microg/mL; P=.003). We recommend adjusting the posology of doxycycline to achieve a serum concentration of at least 5 microg/mL.