Maximising the value of blood cultures

Blood cultures give a critically important window on bacterial and fungal sepsis, providing direct patient treatment guidance and reliable antimicrobial susceptibility data that are used to construct cumulative antibiograms and empirical treatment guidelines.  They also help to rule out serious bacterial infection, provided that collection occurs prior to antibiotic treatment and an adequate patient sample is taken.

Here are some golden rules:

  1. Enhance blood culture specificity – collect cultures in a manner that reduces contamination (aim for < 3% of culture sets). A false positive (contamination) result impedes detection of a pathogen. Cultures collected via pre-existing central lines are more prone to false positive results. Do not collect cultures from in situ peripheral lines.
  2. Optimise sensitivity of bloodstream infection detection by always collecting two separate blood culture sets at each sepsis episode (commercial systems- 40 mLs total for adults, 10 mLs of blood for each bottle, avoiding over or underfilling)
  3. Always discuss a positive result (the initial Gram stain or culture) with a clinical microbiologist. Microbiologists in Pathology North deal with many positive cultures per day and can provide useful advice as to the potential meaning of a positive culture, whether antibiotics are needed and which are the best drugs and dose to use.
  4. Learn about the local epidemiology of bloodstream infections, including contamination rates – confer with one of the clinical microbiologists in Pathology North or the Director of Infection Prevention Service, Dr Ferguson.


Below are pictured two common types of commercial blood culture media (one aerobic and anaerobic bottles makes a set for an adult; children/infants – usually 1 special aerobic bottle used that accommodates a smaller blood inoculation – 1 to 3 mL) :


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