A systematic review of antimicrobial susceptibility testing as a tool in clinical trials assessing antimicrobials against infections due to gram-negative pathogens.

Background: Antimicrobial susceptibility testing (AST) is the standard of care for treating bacterial infections. In randomized clinical trials of new antimicrobials, AST might not be performed or reported in real time.

Objectives: To determine local, real-time laboratory AST performance, its usage in the trial flow, quality control (QC) of the local testing, central AST performance and the effect of using AST categorization on the trials' primary outcomes.

Data Sources: We systematically searched PubMed, Embase, PsychINFO and Web of Science.

Eligibility Criteria: We included registered randomized controlled trials published in journals between January 2015 and December 2019.

Participants And Interventions: We included trials comparing different antibiotics for the treatment of infections caused predominantly by Gram-negative bacteria.

Methods: Primary outcomes for different trial populations were extracted and differences between trial arms were compared for patients with infections caused by susceptible versus non-susceptible bacteria. Results are described narratively.

Results: Of 32 randomized trials, 25 trials reported that local AST was performed, 1312 reported the local laboratory AST methods, no trial reported QC, but post-hoc referral for AST at a reference laboratory was common. Patients' outcomes were superior when patients with infections due to susceptible and non-susceptible pathogens were compared post hoc (median difference 14%, interquartile range 8%-24%) in trials allowing this comparison (seven antimicrobials), except for colistin, where 14-day mortality was 9% higher when patients were treated with colistin for colistin-susceptible versus colistin-resistant carbapenem-resistant Acinetobacter baumannii. When excluding patients with pathogens that were non-susceptible to either antimicrobial in the trials, the difference in the primary outcome between the trial arms was reduced in five out of six trials.

Conclusions: Trials should perform AST to guide patient inclusion or exclusion from the study and consider the impact of the central laboratory susceptibility results on the study outcomes when using post-hoc reference testing.