Reliability of the E-test and VITEK 2 Compact for Tigecycline Susceptibility Testing in Clinical Acinetobacter baumannii Isolates Compared With Broth Microdilution

Background: The growing incidence of multidrug-resistant Acinetobacter baumannii (MDR AB) has left very limited therapeutic options. Tigecycline, the first antibiotic in the glycylcycline class, is approved by the U.S. Food and Drug Administration (US FDA) for the treatment of complicated intra-abdominal infections and complicated skin and skin structure infections. This is one of the few last-resort options for infections caused by MDR AB. However, there is a lack of officially available breakpoints for Acinetobacter baumannii against tigecycline. Objective: This study aims to determine the tigecycline susceptibility of clinical A. baumannii isolates by broth microdilution (BMD), E-test, and VITEK 2 Compact, and to evaluate the performance of E-test and VITEK 2 Compact against BMD as the reference standard. Methodologies: A total of 150 clinical isolates of A. baumannii from sputum, blood, urine, pus, and sterile body fluids identified by VITEK 2 Compact from May 2019 to June 2021 were included in this study. Tigecycline susceptibility was determined by BMD, E-test (minimum inhibitory concentration (MIC): 0.016-256 µg/mL), and VITEK 2 Compact, with MICs interpreted using EUCAST (European Committee on Antimicrobial Susceptibility Testing) 2020 Enterobacterales breakpoints. Result: The proportion of isolates categorized as susceptible by BMD was 90.67%, with MIC50 as 0.125 µg/mL, whereas MIC50 was higher for E-test and VITEK 2 Compact, having values of 0.75 µg/mL and 1 µg/mL, respectively. MIC90 was found to be 0.5 µg/mL in BMD, whereas higher values of 1 µg/mL and 4 µg/mL were found in E-test and VITEK 2 Compact, respectively. Conclusion: Discordant results have been observed across different susceptibility testing methods as well as using different interpretative criteria. As this study was conducted on a small number of isolates, additional studies are needed to determine an interpretation category for Indian isolates.