Comparative Evaluation of BacT/ALERT 3D and BACTEC Systems for the Recovery of Pathogens Causing Bloodstream Infections

Objective: To compare BacT/ALERT (BTA) and BACTEC 9240 (BAC), two continuously monitoring automated blood culture systems, for the recovery of bloodstream pathogens and standard media available for these systems. Materials and Methods: Blood samples from 100 adults and 50 paediatric patients suspected of having bloodstream infections were inoculated at the bedside into non-vented BTA and BAC standard blood culture bottles and incubated in their respective instruments. The time to growth detection (TD) was recorded for each bottle that became positive. A quantitative assay was also carried out with 5 standard bloodstream pathogens to assess TD of each pathogen as well as the quantity of organisms recovered. Results: A total of 23 isolates representing true infections were recovered by both BTA and BAC bottles, indicating ablood culture positivity rate of 15.3%, 18 (78.3%) by BTA bottles and 13 (56.5%) by BAC. Proteus mirabilis, Pseudomonas aeruginosa and Clostridium perfringens were recovered only by the BTA system. The average TDs were 19.0 and 24.6 h for BTA and BAC, respectively. Analysis of the quantitative growth of known pathogens in both systems was more or less the same for Staphylococcus aureus, Escherichia coli and P. aeruginosa but slightly different for Haemophilus influenzae and Streptococcus pneumoniae. The anaerobic bottle ofthe BTA did not support the growth of H. influenzae below an inoculum of 1010 CFU/ml whereas the BAC did so at a lower inoculum of 108 CFU/ml. TD for S. pneumoniae in the BTA was about half of that in the BAC. Conclusions: The BTA system appears to be more efficient in detecting common bloodstream pathogens asa higher inoculum is needed for the BAC system to detect the same organism.