AM report PEARLS 2/3/16: polymicrobial blood cultures

Take-home PEARL: positive cultures from any drain or wound is always hard to interpret! Could be false positive from colonization.


Polymicrobial blood cultures

  • In an older (1989!) paper, they discuss things to consider to judge the significance of a positive blood culture
    • Consider the organisms! In general – gram positives were more likely to be contaminants than gram negatives
    • “That 48% of viridans strep and 25% of S aureus isolates were contaminants is somewhat disturbing, since isolation of these bacteria from blood cultures without an obvious source always raises the possibility of endocarditis.”
    • Other things to consider are summarized in a nice table in the article, and include things like veracity, duration, pattern, clinical severity, CFU, lethality, site of acquisition
  • When in doubt, draw more cultures! 1 out of 2 +BCx is harder to interpret as contaminant than 1 out of 4.
  • Consider the conditions in which the cultures were drawn! (did the provider drawing the cultures use sterile technique?)
  • Found an article from 2010 in Academic Emergency Medicine on characteristics and outcomes of polymicrobial bloodstream infections. Here are some highlights on a study of 112 episodes of polymicrobial BCx (note the limitation of this being from one single institution in Taiwan):
    • 78% had 2 species isolated (the rest had 3)
    • Hospitalization within past 90 days was the strongest RF
    • GI infection and resp infection most common
    • Organisms: 95% gram neg, 46% gram positives
    • Mortality rate 21%-63% (2x higher than those with monomicrobial infections), reportedly related to inappropriate antimicrobial treatment


Enterococcus vs enterobacter:

  • Enterococcus = gram positive – relatively resistant to cell wall active agents (PCN, ampicillin, vanc) and impermeable to aminoglycosides. Quinolones may be useful for enterococcal UTIs but should not be used as monotherapy for bacteremia (serum levels of fluoroquinolones are often close to MIC)
  • Enterobacter = gram negative anaerobe – quinolones are highly active against most strains (though there is emerging resistance). 4th gen cephalosporins and penems are also good agents. Aminoglycosides have good activity but often used in combo therapy.




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