In this episode, I’ll discuss how to prevent unnecessary vancomycin use if your ICU draws routine admission blood cultures.
A recent before-after study published in Critical Care Medicine studied the diagnostic yield of routine blood culture collection for all nonelective ICU admissions (regardless of infection suspicion).
The authors found that drawing routine blood cultures was associated with a 1.5-fold increase of detected bloodstream infection. There was a 4.3-fold increase in the number of contaminated blood cultures, but this was not associated with an increase in vancomycin use in the study ICU.
Physicians may wish to implement this practice of routine blood cultures, but the static rate of vancomycin use may not be generalizable to other centers.
The study authors applied a rigorous algorithm to determine whether a blood culture was a potential contaminate, and this algorithm was included in the supplemental appendix for the article.
The algorithm begins by considering whether the blood culture contains a microorganism that is a skin commensal (i.e. CoNS, Bacillus spp., viridans group Streptococci, Corynebacterium spp., Proprionibacterium spp., Aerococcus spp., and Micrococcus spp.). If no, it was a presumed pathogen. If yes, one of four possible outcomes decide whether the culture was judged to be a contaminate:
1. If there was not a second culture available within one blood culture episode (i.e. 48 hours) with an identical microorganism, the result was considered a contaminate
2. If there was not polymicrobial growth, the result was considered a contaminate
3. If only one bottle was positive, the result was considered a contaminate
4. If the incubation time was greater than 3 days, the result was considered a contaminate
The key metric that the study looked at in regards to increased vancomycin use was in patients without a clinically suspected infection on day 3–5 of their ICU stay. This means that some empiric vancomycin use will occur, as a skin contaminate of coagulase-negative staph for example may initially show as gram-positive cocci before it is identified.
In order to have the best chance of replicating the study results, an evaluation for potentially contaminated blood cultures must occur daily until day 3 has passed, and empiric vancomycin use should be discontinued unless there is another indication or suspected source of infection.
A visual representation of the algorithm used can be found on page 5 of this PDF.
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