| Abstract | Background: The MYSTIC Programme is a global, longitudinal antimicrobial surveillance network of hospitals
that frequently utilize carbapenems. One aspect of the programme is the ability to capture antimicrobial
consumption data from participating institutions. The current report evaluates these relationships for
Enterobacteriaceae and Pseudomonas aeruginosa over the initial 3 year period of the programme in the
USA.
Methods: Between 10 and 15 medical centres participated during 1999–2001, each submitting up to 200 isolates/
year (7003 strains overall). Evaluations of the relationship between drug usage and antimicrobial
resistance in P. aeruginosa and Enterobacteriaceae for the carbapenems (imipenem and meropenem),
cefepime, ceftazidime, ciprofloxacin, gentamicin and piperacillin–tazobactam were determined. Data were
analysed based on: (1) aggregate usage results; (2) medical centre-specific usage compared with resistance
rates; and (3) medical centre-specific usage results compared with yearly changes in resistance rates (ΔR).
The parameter of drug usage was the defined daily dose (DDD)/100 patient days calculated from total grams
administered, using WHO definitions.
Results: Resistance (1999–2001) among Enterobacteriaceae did not change significantly for β-lactams, but
tended to increase slightly for gentamicin (+1.1%) and ciprofloxacin (+3.1%). P. aeruginosa resistance rates
(1999–2001) for gentamicin (+9.0%) and ciprofloxacin (+10.2%) increased, in contrast to a significantly
decreased resistance rate for meropenem (–7.7%). Formulary-use changes were noted: increased meropenem
and ciprofloxacin use and decreased consumption for imipenem, aminoglycosides, ceftazidime and
cefepime. Aggregate ciprofloxacin DDD/100 days rates were directly related (+3.3 DDD) to Enterobacteriaceae
and P. aeruginosa resistance changes, whereas among P. aeruginosa, usage and resistance were
inversely correlated for gentamicin (–3.8 DDD; +9.0% resistant). Medical centre-specific antimicrobial usage
calculations did not demonstrate a correlation to rates of resistance (r = –0.38 to 0.61) or yearly changes in
resistance rates (r = –0.56 to 0.43).
Conclusions: The availability of aggregate USA medical centre antimicrobial usage data enabled us to identify
several important trends in the incidence of resistance among P. aeruginosa and Enterobacteriaceae: (1)
increased use of ciprofloxacin associated with a higher resistance among Enterobacteriaceae; and (2) a correlation
between ciprofloxacin categories of resistance and levels of resistance to other antimicrobial
classes in P. aeruginosa. Medical centre-specific antimicrobial usage and resistance did not demonstrate
direct statistical relationships, and require a continued search for other monitoring methods that can better
identify antimicrobial/environmental factors that lead to resistance.
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