Background: In 2020, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) redefined the “I” category in antimicrobial susceptibility testing (AST) to “susceptible, increased exposure,” aiming to enhance antibiotic therapy by encouraging the use of targeted, narrow-spectrum antibiotics at higher doses instead of broader-spectrum antibiotics at standard doses. By July 2022, Belgian laboratories were encouraged to adopt these changes, categorizing most antipseudomonal drugs as “I” while retaining meropenem as “S.” This study assessed the impact of this redefinition on antibiotic selection for Pseudomonas aeruginosa infections in Belgian hospitals and identified factors associated with meropenem prescriptions.

Methods: Between November 2023 and July 2024, a multicentric retrospective observational study was conducted across Belgian hospitals. Data were collected via a Redcap tool, including hospitalized adults treated for wild-type P. aeruginosa monobacterial infections during a six-month period pre- and post-implementation of the new EUCAST definitions. Data included hospital-level factors (characteristics, AST reporting strategies, campaigns) and patient-level factors (epidemiological, microbiological, clinical, and therapy details). The primary outcome was the proportion of meropenem prescriptions post-susceptibility testing. Multilevel logistic regression identified factors associated with meropenem prescription.

Results: Among 1,705 patients (850 pre- and 855 post-implementation) from 37 acute care hospitals (35.9% national participation), meropenem prescriptions increased significantly for those prescribed this drug (p=0.027) post- EUCAST update, from 4.6% (39/850) to 7.1% (6⅛55). This increase occurred in 45.9% (17/37) of participating hospitals. Six factors were significantly and independently associated with meropenem prescription. Patients treated post-implementation had nearly double the odds of receiving meropenem (OR=1.99, IC 95% [1.24,3.22]). Sharing written information exclusively post-shift was associated with a markedly higher likelihood of meropenem use (OR=9.76, IC 95% [1.73,54.96]). Other factors linked to higher meropenem prescription included sepsis (OR=2.07, IC 95% [1.16,3.72]), Extended Spectrum Beta-Lactamase (ESBL) Enterobacteriaceae history (OR=2.98, IC 95% [1.12,7.92]), and allergy to penicillins, cephalosporins, or fluoroquinolones (OR=4.48, IC 95% [2.22,9.06]). Selective AST reporting for meropenem decreased its prescription likelihood (OR=0.16, IC 95% [0.07,0.38]).

Conclusions: The 2020 EUCAST criteria led to increased meropenem use for P. aeruginosa monobacterial infections in Belgium. While patient’s current and past severe infectious state and allergies to first-line antipseudomonal partly explain the prescription of meropenem, Findings also highlight the role of selective AST reporting in reducing meropenem use and presume the importance of prescriber education and adhesion prior to implementing significant change in their clinical practice. This study suggests that targeted stewardship programs can support appropriate antimicrobial use.