Effect of overnight storage of blood culture bottles on bacterial detection time in the BACTEC 9240 blood culture system

Background/purpose: Identifying the pathogens present in blood stream infections is crucial to initiate appropriate antimicrobial therapy and avoid morbidity and mortality. The aim of this study was to evaluate the effect of overnight storage of aerobic and anaerobic BACTEC 9240 blood culture bottles on the detection time for common pathogens.

Methods: From November 2007 to July 2008, a total of 2,105 isolates were positively detected using the BACTEC 9240 system. The time to positive detection (TTD) was calculated by subtracting the time of receipt in the laboratory from the time required to detect a positive culture. The mean TTD values were calculated using the TTD value of the first positive culture bottle only. Overnight delay at the National Cheng Kung University Hospital, Taiwan was 15 hours (from 5 pm to 8 am).

Results: Of the 2,105 total isolates, 972 (46.1%) were Gram-positive bacteria, 1,024 (48.6%) were Gram-negative bacteria and 109 (5.1%) were fungi. Among the top 10 pathogens, 24.7% grew only in the aerobic bottle and 15.1% in the anaerobic bottle, including Staphylococcus spp., Enterococcus faecium, Enterobacteriaceae, and Gram-positive bacilli. Due to the overnight delay in loading a blood culture bottle into the instrument, for most of the pathogens (including Staphylococcus spp. and Enterobacteriaceae), a decrease in TTD by <or= 4.4 hours was observed. An increase in TTD by 20.8 hours was observed for Gram-positive bacilli. We also found that the difference between TTD in aerobic versus anaerobic bottles during the day was higher in coagulase-negative staphylococcus (12 hours) and lower in Escherichia coli and Staphylococcus aureus (< 2 hours). TTD was longer than 72 hours in 20.5% of Gram-positive bacilli and 7.3% of Candida albicans.

Conclusion: No difference in the TTD of major pathogens was observed in bottles processed during the day and after overnight delay, suggesting that the delayed entry of the blood culture bottle into the instrument may affect the detection time. Since high numbers of facultative anaerobes were detected in anaerobic bottles only, use of a single aerobic bottle might have a detrimental effect on the clinical therapy outcome.