Isolation of bacterial cerebrospinal fluid culture contaminants at a major military medical center

In recent decades, bacterial meningitis rates have decreased secondary to the success of routine vaccinations. Ironically, the decreased incidence may contribute to the challenge of establishing accurate and timely diagnoses. Studies have suggested that in immunocompetent patients with normal cerebrospinal fluid (CSF) white blood cell counts (WBC), positive CSF cultures may be disregarded as presumed contaminants, making the initial CSF WBC increasingly relevant. This single-institution retrospective study sought to integrate clinical data with positive cultures in an era when CSF contaminants may be more commonly isolated in culture than true pathogens. A total of 7715 adult and pediatric CSF samples from 1995 to 2009 were obtained at a major military medical center. Clinical and laboratory data from 121 positive bacterial cultures were reviewed. Our bacterial CSF contamination rate (false positives) was 0.91% (70/7715). True-positive (TP) CSF cultures totaled 51 (0.66%). Among TPs, 16% (8/51) demonstrated normal CSF cell counts. The notably low 15-year CSF contamination rate of 0.91%, suggests that positive cultures are likely to represent true infection in our institution. We believe efforts to decrease the contamination rate are among the most cost-effective, while targeted clinical re-evaluation for all patients with positive CSF cultures remains vital. In light of this data, a targeted approach to re-evaluating positive cultures while incorporating the clinical context remains prudent.