Effect of different strategies for excluding duplicate cultures on the correlation between hospital resistance rates and antibiotic consumption

Introduction Studies may underestimate the impact of antibiotics on bacterial resistance when correlating hospital antibiotic use with resistance rates (RRs) that exclude duplicate cultures as duplicates usually include more resistant isolates. Comparing correlations between antibiotic consumption and RRs resulting from different strategies for excluding duplicates could help explore how their exclusion affects such correlations.Methods We obtained antibiotics consumption and Pseudomonas aeruginosa susceptibility data from 2017 to 2021 for seven antibiotics and for carbapenems as a group in a university hospital. We calculated RRs using seven different time criteria for excluding duplicates. We assessed the correlations of antibiotic consumption to the same-year and next-year RR rates for the three most distinct rates.Results Duplicate cultures represented 53.45% of total cultures. RRs were higher when duplicates were included. We compared RRs resulting from excluding all duplicates, excluding duplicates monthly or admitting one culture per day. All antibiotics except meropenem showed a correlation with same-year RRs, either positive or negative, whereas all antibiotics showed a positive correlation with next-year RRs. For same-year and next-year correlations, the criteria with fewer duplicates (and therefore fewer resistant strains) found more correlations. However, the inclusion of duplicates taken at least 1 month apart found the most correlations. Admitting one culture per day found the fewest correlations.Conclusions Excluding duplicates from RRs affects the correlation of antibiotics consumption with RRs in P. aeruginosa. Including at least some duplicate cultures in correlation analyses, such as those taken 1 month apart, should be considered.