Effect of blood volume in automated blood culture of the BACT/ALERT 3D system on isolation rate and time to positivity of pathogens, in a tertiary care hospital, South India

Aim: To determine the appropriateness of blood volume in automated blood culture bottles of the BACT/ALERT 3D system, its effect on isolation rate and time to positivity of pathogens. Materials and Methods: The study was carried out in a tertiary care hospital in South India from 1st March 2018 to 31st August 2018. Automated blood culture bottles sent to Microbiology laboratory for blood culture were included in the study. The bottles were weighted by electronic weighing balance. The filled blood culture bottles received at microbiology laboratory were re-weighed and was categorized into appropriately filled and inappropriately filled bottles. Frequency of appropriately filled bottles and inappropriately filled bottles was expressed in percentage. Result: A total of 8740 blood culture bottles were included in the study, out of which the blood volume was found to be suboptimum in 3978 bottles (45.5%), optimum in 3498 bottles (40%) and overfilled in 1264 bottles (14.5%). There was an increase in the number of blood culture bottles with optimum blood volume from 35.1% in March to 58.12% in August. The average TTP for true pathogens was found to be 16.72 hrs. with optimum blood volume, 17 hrs. with overfilled blood volume and 21.5 hrs. with sub-optimum blood volume. The isolation rate for true pathogens was found to be 88.62% with optimum blood volume, 86.43% with overfilled blood volume and 70.01% with sub-optimum. The false positive rate was found to be 4.1% with overfilled blood volume, 1.6%with sub-optimum and 1.4%with optimum blood volume. Conclusion: Through this study, the correlation between the blood volume with the isolation rate, time to positivity and false positivity of blood culture isolates was established. Keywords: Blood culture, Blood volume, BACT/ALERT 3D, Time to positivity, Isolation rate.