P19 Impact of the introduction of the EUCAST 2020 susceptibility interpretation rules for <i>Pseudomonas aeruginosa</i> on selection of antibiotic therapy

Abstract Background In 2020, EUCAST changed the interpretation of WT Pseudomonas aeruginosa susceptibility to the first-line agents ciprofloxacin, piperacillin/tazobactam and ceftazidime from susceptible (S) to susceptible, increased exposure (I). The breakpoint changes were implemented at Cambridge University Hospitals (CUH) in March 2020, initially with no explanatory notes, and from June 2020 with the addition of an interpretative comment. Objectives To study the impact of implementing the EUCAST susceptibility interpretation of WT P. aeruginosa on carbapenem prescribing at CUH. Methods In this service evaluation, we analysed all clinical isolates of P. aeruginosa between 1 January 2020 and 30 September 2020. Only WT (fully susceptible isolates) were included. We retrospectively collected epidemiological data, clinical background, treatment information and Microbiology involvement and analysed this data to identify prescription rates and factors influencing meropenem prescription in P. aeruginosa infections, comparing three periods: Period 1 (pre-transition), Period 2 (transition) and Period 3 (post-transition). Results In total, 104 patients were included in the study. Meropenem use across all three periods was similar (17.6%, 18.4% and 12.5%, P=.799). Selective susceptibility reporting of meropenem susceptibility differed between periods, with a transient increase in reporting during period 2 (11.8%, 36.1% and 6.3%, respectively [P=.004]). Reporting of susceptibility to meropenem was associated with increased odds of meropenem prescribing (OR 4.3, 95% CI 1.40–13.7, P=.011). Conclusions The changes to the EUCAST breakpoints for P. aeruginosa did not have a significant impact on carbapenem prescription at CUH. This in part could be explained by selective susceptibility reporting of results.