Evaluation of three different methods for susceptibility testing of gentamicin in carbapenem resistant Enterobacterales.

Carbapenem-resistant Enterobacterales (CRE) have become a growing problem worldwide in recent years. Options for the treatment of CRE are limited and one of these options is gentamicin. For this reason, gentamicin susceptibility should be properly determined. In a recently reported study, it is recommended to review the results of automated systems for assessing gentamicin susceptibility in carbapenem-resistant isolates. In this study, we aimed to determine gentamicin susceptibility using three different methods and compare the methods. The study included 107 CRE isolates from different samples. Gentamicin susceptibility was determined using Vitek 2 Compact (bioMérieux, France), Microscan Walkaway Plus (Beckman Coulter, USA) automatic systems, and disk diffusion (DD) method. The broth microdilution method (BMD) was used as reference method. Minor, major, and very major errors and categorical agreement rates were determined for each method. Aminoglycoside-modifying enzymes (aac(6')Ib and aph(2″)Ia) were assayed in discrepant isolates. According to BMD results, 90.7%, 1,8 %, and 7.5 % of the isolates were determined as susceptible, intermediate, and resistant to gentamicin, respectively. Compared to the results of the BMD for detecting gentamicin susceptibility, disk diffusion method showed the highest categorical agreement (98.1%), and Vitek 2 Compact showed the lowest categorical agreement (90.6%). The very major error rates were determined 7.5%, 0.9%, and 0.9% for Vitek 2 Compact, Microscan Walkaway Plus, and DD method, respectively. In addition, aac(6')Ib and aph(2″)Ia genes were detected in 8 discrepant isolates. For gentamicin susceptibility, the DD showed the most compatible results. The DD can be used as a reliable method for determining gentamicin susceptibility. Compatibility of automated systems with BMD was acceptable, although lower than DD. The discrepancies detected in the Vitek 2 Compact results could be due to the presence of aac(6')Ib and/or aph(2″)Ia aminoglycoside-modifying enzymes.