Influence of antibiotic therapy prior to admission on the efficacy of classical methods for the diagnosis of meningococcal disease.

To assess the influence of preadmission antibiotic therapy on the results of the classical methods for bacteriological confirmation of meningococcal disease (MD).Retrospective study of the MD cases diagnosed in the "St. Parascheva" Universitary Clinical Infectious Diseases Iaşi between 1994 and 2011.The etiological diagnosis was made by identifying the meningococcus in the CSF (cerebrospinal fluid) in 71.9% of the 323 patients and by blood culture in 8%. Preadmission antibiotic therapy received 39% of the patients, thus the sensitivity of test was significantly reduced: direct examination from 64.6% to 43.2% (p < 0.001), cultures from 55.9% to 27.2% (p < 0.001), and latex-agglutination from 84.6% to 58.8% (p = 0.003). The rate of positive CSF decreased from 82.1% to 56% (p < 0.001). Preadmission antibiotic therapy significantly increased the ratio of cases in which meningococcus was not detected in CSF by any of the classical methods (44% compared to 17.9% in the cases without prior treatment). The proportion of cases in which meningococcal isolation was done by two methods decreased from 38.5% to 19.2%, and of those by all three methods from 16.9% to 5.6% (p < 0.001). Preadmission antibiotic therapy also decreased the rate of positive blood cultures from 14.7% to 3.5% (Fisher's exact test, p = 0.009).Antibiotic treatment prior to admission significantly decreases the percentage of patients with MD in which meningococcal isolation can be done; this requires the use of a more sensitive diagnosis method (ex. qPCR).