Aims
Prompt treatment reduces bacteraemia-associated mortality. Blood cultures may be collected after antibiotic administration. Early bacteraemia detection is dependent on the antibiotic neutralising ability of absorbent resins in culture media. This study evaluated the performance of two blood culture systems by comparing the time to detection (TTD) of seeded vials in the absence and in the presence of antibiotics.
Methods
BacT/Alert FAN® Plus media and BACTEC™ Plus media were inoculated with sheep blood and microbial suspensions before incubating into the respective automated systems: BacT/Alert® VIRTUO™ (BacT VIRTUO) and BACTEC™ FX (BactecFX). Testing was performed in duplicate for 39 unique bacterial strains (n=78 vials).
To evaluate antibiotic absorption, various concentrations of vancomycin or ceftriaxone were inoculated into seeded vials. Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, Enterococcus faecalis and Enterococcus faecium were used for vancomycin evaluation. Enterobacteriaceae (1 resistant, 10 sensitive strains) and Aeromonas hydrophilia were used for ceftriaxone evaluation. Duplicate testing was performed for each organism/antibiotic concentration (n=58 vials).
Results
In the absence of antibiotics, 77 (99%) of tested vials had a faster TTD for BacT VIRTUO (median TTD of 9.4 hours) compared to BactecFX (11.2 hours). Shorter mean TTD range was observed for BacT VIRTUO (5.4 - 36.7 hours) compared to BactecFX (7.55 - 51.3 hours). BacT VIRTUO had an earlier detection rate for all tested organisms (n=24 vials) in controls, and in vials with vancomycin trough and peak levels (median TTD of 10.7 hours, 12.0 hours and 15.2 hours respectively) compared to BactecFX (12.4 hours, 16.3 hours and 20.6 hours respectively). At low ceftriaxone concentrations (5, 10, 15 and 30μg/mL), BactecFX (55.6% recovery rate; median TTD of 11.7 hours) is more efficient in recovering Gram-negative bacilli than BacT VIRTUO (22.2%, 6.0 hours). Both systems were unable to recover organisms at high ceftriaxone concentration (120μg/mL).
Conclusions
BacT VIRTUO has a shorter TTD than BactecFX in the absence of antibiotics and in the presence of vancomycin. Suboptimal recovery rate was observed in both systems when ceftriaxone was added.