Introduction to the measurement of the avidity of anti- Coxiella burnetii IgG in the diagnosis of Q fever.

Due to the atypical serological profile of some patients with primary Q fever infection who do not develop IgM against Coxiella burnetii, we developed an avidity test to distinguish recent or past infections. We tested 39 serum samples by immunofluorescence with conventional assay and after urea treatment from 26 patients at different stages of the disease. We observed a strong avidity in the 15 serum samples from patients with infections of >6 months and a low avidity for sera from patients with recent infections. A complete denaturation of the antibodyantigen complex was observed for patients for whom the time since the beginning of infection was <1 month and a mean of 2.06 +/- 0.54 lowered titers when the infection was less than 3 months old. That was statistically significant compared to sera from patients with infections of greater than 6 months (mean 0.20 +/- 0.41) and with infections between 3 and 6 months (mean, 1.17 +/- 0.41) (P = 0.0022 and P < 0.0001, respectively). These results were visualized by Western blotting. We concluded that high avidity (<= 1 lowered titer) ruled out infection during the last 6 months and that complete denaturation was related to an infection which had occurred within the previous 3 months. Between these two situations, the avidity test is inconclusive. We suggest using an avidity test for atypical Q fever serology that could be misclassified as residual antibodies (IgG against C. burnetii detected without active or recent infection) and for pregnant women risking obstetrical complications. This new test will dramatically improve the diagnosis and management of patients with Q fever.