Effect of prior antibiotic therapy on concentrations of bacteria in CSF.

A prospective study was done to determine the effect of prior antibiotic therapy on concentrations of bacteria in CSF at the time of diagnosis. Concentrations of Haemophilus influenzae type b in CSF of partially treated patients were significantly smaller than in CSF of untreated patients. This was also true of patients with meningococcal meningitis, but no difference was observed between patients with pneumococcal meningitis. The identification of H influenzae by Gram stain of CSF was significantly decreased by prior antibiotic therapy. Measurable levels of antibiotic (penicillins, aminoglycosides) were present in CSF of ten of 23 patients. Two partially treated patients were diagnosed by detection of meningococcal antigen in CSF by counterimmunoelectrophoresis although cultures of CSF were sterile. Both of these patients had measurable levels of penicillin in CSF. These data indicate that prior antibiotic therapy significantly decreases the concentration of H influenzae type b and meningococcus in CSF and therefore decreases the usefulness of CSF Gram stain. Partial therapy may sterilize the CSF of some patients.