Change in antipseudomonal carbapenem susceptibility in 25 hospitals across 9 years is not associated with the use of ertapenem

Background: Concern remains that ertapenem use may promote cross-resistance in Pseudomonas aeruginosa to antipseudomonal carbapenems (APCs). This study extends our earlier multicentre investigation of this relationship by an additional 3 years. Methods: Use density ratios (UDRs) for ertapenem, APCs, aminoglycosides, fluoroquinolones and non-carbapenem beta-lactams were derived from purchase data for 3 years pre-adoption and up to 6 years post-adoption of ertapenem at 25 hospitals. Hospital antibiograms in corresponding years yielded APC susceptibility data. Mixed model repeated measures ANOVA explored associations between 9 year repeated APC susceptibility and ertapenem UDR while controlling for all other classes. Results: All 25 sites had 4 years of post-adoption data, with 22 of 25 reporting 5 years and 18 of 25 reporting 6 years. Ertapenem UDR rose steadily once adopted, with a mean UDR of 7.27 in year 4 and a mean UDR of 15.93 in year 9. APC UDR increased initially (from 10.39 in year 1 to a peak of 18.77 in year 6) and then declined to 15.27 in year 9. By year 9 ertapenem and APC use were similar. Among other classes, fluoroquinolone UDR increased notably (year/mean UDR): 1/303.84; 4/174.38; and 9/423.32. Mean APC susceptibility declined from 85.4% in year 1 to 81.0% in year 9; this change across time was not significant (P=0.99). Change in 9 year APC susceptibility was not associated with ertapenem UDR (P=0.54), while controlling for all other antibiotic classes (all showed no association at P>0.5). Conclusions: While controlling for utilization of other antibiotic classes, we found no association between change in APC susceptibility and ertapenem use.