False aminoglycoside resistance in Enterobacterales and non-fermenters by an automated testing system: a descriptive study

The majority of antimicrobial susceptibility testing (AST) in Dutch medical microbiology laboratories (MMLs) is performed using the automated system VITEK2 or Phoenix. One of the Phoenix users noted a high percentage of gentamicin resistance in Proteus mirabilis compared to national resistance data. We therefore performed a descriptive analysis comparing gentamicin and tobramycin minimum inhibitory concentration (MIC) distributions for Enterobacterales and non-fermenters as measured using VITEK2 and Phoenix. Routine susceptibility data from 5 MMLs using Phoenix and 31 using VITEK2 with complete data in the Dutch national surveillance system for antimicrobial resistance from January 2016 to December 2021 were included. A panel of 12 discrepant isolates was sent to Becton Dickinson for confirmation. In general, Phoenix measured higher MICs, with discrepancies being most prevalent for Proteus, Providencia, Morganella, Serratia, and Acinetobacter species (borderline susceptibility for gentamicin ranging from 1% for VITEK2 to 67% for Phoenix systems), and less, but still clinically relevant, for Escherichia coli (1%-12%). Furthermore, we observed a yearly increase in resistance for Proteus and Providencia species measured by Phoenix. Similar discrepancies were found for tobramycin. The company confirmed our findings on all strains. Significantly more false aminoglycoside borderline susceptible and resistant Enterobacterales were found using Phoenix compared to the VITEK2 system. These findings should be taken into consideration in the development of clinical treatment guidelines for patients with sepsis.IMPORTANCEAntimicrobial sensitivity data are important to guide antimicrobial therapy. In microbiological laboratories, routine sensitivity measurements are typically performed with automated testing systems such as VITEK2 and Phoenix. Using data from the Dutch national surveillance system for antimicrobial resistance over a 6-year period, we found that the measured minimum inhibitory concentrations for aminoglycosides in Enterobacterales and non-fermenters were too high for the Phoenix system. In addition, we observed a yearly increase in resistance for several species measured by Phoenix. These findings might have consequences for clinical treatment of patients with sepsis. Antimicrobial sensitivity data are important to guide antimicrobial therapy. In microbiological laboratories, routine sensitivity measurements are typically performed with automated testing systems such as VITEK2 and Phoenix. Using data from the Dutch national surveillance system for antimicrobial resistance over a 6-year period, we found that the measured minimum inhibitory concentrations for aminoglycosides in Enterobacterales and non-fermenters were too high for the Phoenix system. In addition, we observed a yearly increase in resistance for several species measured by Phoenix. These findings might have consequences for clinical treatment of patients with sepsis.