Evaluation of Positive Rate of Aerobic BacT/Alert Blood Culture Bottles by Antibiotic Usage and Inoculated Blood Volume

Park, Do Sim; Lee, Young Jin; Yoo, Su Jin; Cho, Ji Hyun

Abstract

BACKGROUND
Inoculated blood volume and residual antibiotic effects are the key variables that influence successful blood cultures especially in patients receiving antibiotics. In our hospital we have inoculated BacT/Alert blood culture bottles with 5 mL of blood, but recent reports about the BacT/Alert system recommend an inoculation of up to 10 mL of blood.
METHODS
We studied about 931 paired aerobic blood culture sets and each set was inoculated with 3 mL (3 mL bottle) and 7 mL (7 mL bottle) of blood. The positive rates, the yield and speed of detecting microorganisms in the culture bottles were compared to the specimen information of antibiotic usage. We also analyzed the antibiotic effect on each set of blood cultures by inoculation with standard microorganisms (E. coli ATCC 25922 and S. aureus ATCC 25923) in the used bottle sets.
RESULTS
Positive cultures were obtained from 98 sets (10.5%) and 161 organisms grew. Of 931 blood culture bottle sets, 744 (79.9%) were obtained from patients on antibiotic therapy. The positive rate of blood culture bottles from patients receiving antibiotic therapy was significantly lower than those from patients who were not receiving antibiotic therapy (P<0.05). Irrespective of antibiotic usage before using the blood cultures, the 7 mL bottle showed a better yield and speed of detecting microorganisms than the 3 mL bottles, but it was not statistically significant (P>0.05). When culture bottles with no growth (from patients receiving antibiotic therapy) were used, no differences were found in detection time and positive rates between 3 mL and 7 mL blood culture bottles that were artificially seeded with standard E. coli or S. aureus strains.
CONCLUSIONS
Although antibiotic usage rate before using the blood cultures was high in our hospital, the 7 mL bottle showed a better yield and speed of detecting microorganisms than did the 3 mL bottle irrespective of antibiotic usage before using the blood cultures. This data suggests that blood volume elevation (from 5 mL up to 10 mL) can be recommended for BacT/Alert Blood culture systems.