Cerebrospinal fluid broth culture isolates: their significance for antibiotic treatment

Enriched broth medium is routinely used as a supplement for agar plate culture of cerebrospinal fluid (CSF). To assess the clinical utility of broth cultures, 151 consecutive CSF bacterial and fungal isolates obtained from 91 patients were retrospectively reviewed for the effect of results on treatment. Treatment decisions associated with individual CSF specimens for which isolates were recovered from thioglycollate broth only were compared with the treatment decisions associated with CSF specimens for which isolates were recovered by agar plate culture. Treatment was defined as initiation of or change in antimicrobial therapy based on the reporting of CSF culture isolates. Thirty-six (24%) of the 151 isolates were recovered in broth only. Three (8%) of these 36 isolates (from 34 patients) resulted in treatment with antimicrobial agents; however, 2 of the 3 treated isolates (Candida tropicalis, Proteus mirabilis) were recovered from a second CSF specimen in agar plate culture within 24 hours. Thus, only a single isolate (3%; Staphylococcus epidermidis) was treated based solely on a positive broth culture result. In contrast, 60 (52%) of the 115 isolates recovered in agar plate culture from 23 (40%) of 57 patients were treated (staphylococci, 28; gram-negative bacilli, 14; Cryptococcus neoformans, 10; Streptococcus pneumoniae, 3; Streptococcus sanguis, 1; other, 4). We conclude that treatment with antimicrobial agents based on isolates recovered from CSF specimens in broth culture alone is infrequent and infer from the data that the use of CSF broth cultures contributes little to treatment decisions.