Time-to-positivity of aerobic blood cultures by using BacTec 9120 blood culture system

Abstract number: 1733_433

Fokas St., Fokas Sp., Altouvas G., Tsironi M., Kaptanis S., Kalkani M., Dionysopoulou M.

Objectives: The aim of this study was to determine the time to detection of positive aerobic blood cultures with the BACTEC 9120 system.

Methods: We retrospectively analyzed the data of 3250 aerobic blood cultures incubated for a period of 7 days in the BACTEC 9120 system from June 2004 to June 2006. Blood samples were drawn from peripheral veins and were inoculated into two aerobic/F bottles for each blood culture. Whenever there was a sign of positive culture the detection time, measured in hours, was documented for each bottle. Negative blood cultures from patients with clinical evidence of brucellosis were subcultured blindly.

Results: A total of 247 (7.6%) positive cultures were detected and 62 (1.9%) of them were clinically insignificant. Gram-negative organisms were recovered more frequently than Gram-positives in true bacteraemias (78%, 144/185 strains versus 22%, 41/185 strains) while CoNS were the most common contaminants. The mean detection time for all microbes (435 isolates) was 24.13 h (range 2.88 h to 122 h) and 72% (313/435) of them were detected within the first 24 h of incubation. The mean detection times for the Gram-negative and Gram-positive bacteria were 31.08 h and 21.9 h respectively (E. coli 9.3 h, Proteus spp. 9.2 h, Klebsiella spp. 9.2 h, P. aeruginosa 14.2 h, Brucella melitensis 65.39 h, S. aureus 15.5 h, CoNS 23.6 h, S. pneumoniae 10 h, Enterococcus spp. 19.8 h, Streptococcus spp. 36.6 h, Candida spp. 15.6 h).

Conclusions: The recovery time of Enterobacteriaceae (9.2 h) was significantly shorter of Gram-positive bacteria and 85% of Enterobacteriaceae were isolated within the first 24 h. All pathogenic microbes, including Brucella spp. were recovered within 5 days of incubation and the detection times of clinically significant isolates were shorter than contaminants. A protocol of 5-days incubation period is possible to apply with the BACTEC 9120.