3.4 Culture-guided antibiotics
Three retrospective cohort studies investigated the use of antibiotics
in the context of tracheal aspirate cultures.
Two studies aimed to establish the usefulness of endotracheal aspirate
cultures in guiding choice of antimicrobial therapy. Prinzi et al.
looked at the association between over-reporting of such cultures in
tracheostomised paediatric patients and its subsequent effect on
antimicrobial
prescribing.37Over-reporting was defined according to the American Society of
Microbiology guidelines as reporting organisms not known to be
respiratory pathogens. During the one-year study period, 826
endotracheal aspirate cultures were collected from 448 children. From
these cultures, 310 isolates were identified in tracheostomised
children. Of which, 25 (8%) organisms were over-reported, resulting in
48 days of excess antimicrobial therapy. Cline et al. aimed to assess
the utility of surveillance cultures (routine tracheal aspirate
cultures) in children with tracheostomies in guiding antimicrobial
selection for subsequent
LRTIs.38The study concluded that due to the dynamic nature of the tracheal
microbiome on serial cultures, historical cultures are of little value
to dictate antimicrobial choice in subsequent infections. Indeed, they
report that in over half of cases (n = 36), limiting empirical
antimicrobials to a previous culture result would not cover organisms
isolated on subsequent cultures.
Yalamanchi et al. explored whether microscopic purulence, which is the
quantitative assessment of neutrophils, in positive tracheal aspirate
cultures could be used to predict subsequent antimicrobial use in a
single centre retrospective review39. In
their study cohort of 231 children admitted to intensive care units (81
tracheostomised), there were 361 positive tracheal aspirate cultures, of
which a fifth (22%, n=81) were treated with antibiotics. Using
multivariate logistic regression, they showed microscopic purulence to
be an independent predictor of antimicrobial use, alongside pyrexia and
respiratory failure. However, microscopic purulence was not associated
with clinical symptoms of infection (hypotension, fever, or increased
respiratory support). It should also be noted that this regression model
aimed to predict current antibiotic prescribing practice rather than a
“gold-standard” benchmark of confirmed bacterial infections. As such
it may represent a useful metric in aiding the decision to initiate
antimicrobial therapy, but only in the clinical context of suspected
infection.