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Neonatal blood cultures: Effect of delayed entry into the blood culture machine and bacterial concentration on the time to positive growth in a simulated model

Authors

  • Luke Anthony Jardine,

    1. Grantley Stable Neonatal Unit, Royal Brisbane and Women's Hospital,
    2. Department of Paediatrics and Child Health, The University of Queensland, Royal Children's Hospital and
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  • Barbara Ruth Sturgess,

    1. Department of Microbiology, Royal Brisbane and Women's Hospital, Brisbane, Queensland Australia
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  • Garry Donald Trevor Inglis,

    1. Grantley Stable Neonatal Unit, Royal Brisbane and Women's Hospital,
    2. Department of Paediatrics and Child Health, The University of Queensland, Royal Children's Hospital and
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  • Mark William Davies

    1. Grantley Stable Neonatal Unit, Royal Brisbane and Women's Hospital,
    2. Department of Paediatrics and Child Health, The University of Queensland, Royal Children's Hospital and
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Dr Luke Jardine, Grantley Stable Neonatal Unit, Royal Brisbane and Women's Hospital, Brisbane, Qld 4029, Australia. Fax: +61 7 3636 5259; email: Luke_Jardine@health.qld.gov.au

Abstract

Aims:  To determine if: time from blood culture inoculation to positive growth (total time to positive) and time from blood culture machine entry to positive growth (machine time to positive) is altered by delayed entry into the automated blood culture machine, and if the total time to positive differs by the concentration of organisms inoculated into blood culture bottles.

Methods: Staphylococcus epidermidis, Escherichia coli and group B beta-haemolytic streptococci were chosen as clinically significant representative organisms. Two concentrations (≥10 colony-forming units per millilitre and <1 colony-forming units per millilitre) were inoculated into PEDS BacT/Alert blood culture bottles and randomly allocated to one of three delayed automated blood culture machine entry times (30 min/8.5 h/15.5 h).

Results:  For all organisms at all concentrations, except the Staphylococcus epidermidis, the machine time to positive was significantly decreased by delayed entry. For all organisms at all concentrations, the mean total time to positive significantly increased with increasing delayed entry into the blood culture machine. Higher concentrations of group B beta-haemolytic streptococci and Escherichia coli grew significantly faster than lower concentrations.

Conclusion:  Bacterial growth in inoculated bottles, stored at room temperature, continues although at a slower rate than in those blood culture bottles immediately entered into the machine. If a blood culture specimen has been stored at room temperature for greater than 15.5 h, the currently allowed safety margin of 36 h (before declaring a result negative) may be insufficient.

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