Comparative evaluation of tigecycline susceptibility testing methods for Acinetobacter baumannii and Enterobacteriaceae

The objectives of this study were to recommend suitable susceptibility testing methods for tigecycline for clinical laboratory use and to evaluate differences in tigecycline susceptibility between carbapenem-susceptible and carbapenem-non-susceptible isolates. Broth microdilution (BMD) was used as the reference method to evaluate MIC Test Strip (MTS), agar dilution, VITEK® 2 and disk diffusion testing methods for tigecycline against Acinetobacter baumannii and Enterobacteriaceae. MIC50/90 values (minimum inhibitory concentrations for 50% and 90% of the isolates, respectively) of A. baumannii and Enterobacteriaceae were, respectively, 2/4 μg/mL and 0.5/4 μg/mL by BMD, 1.5/3 μg/mL and 0.5/3 μg/mL by MTS, 2/4 μg/mL and 1/8 μg/mL by agar dilution and 2/4 μg/mL and 2/8 μg/mL by VITEK® 2. Essential agreement for A. baumannii/Enterobacteriaceae detected by MTS, agar dilution and VITEK® 2 methods was 96.0/97.3%, 98.0/97.3% and 94.0/63.9%, respectively. Categorical agreement for A. baumannii/Enterobacteriaceae detected by MTS, agar dilution, VITEK® 2 and disk diffusion methods was 90.0/91.8%, 72.0/93.7%, 62.0/86.5% and 72.0/81.2%, respectively. No very major errors were found for all isolates by the four methods evaluated. Major error rates were produced by VITEK® 2 (for Enterobacteriaceae) and by disk diffusion (for A. baumannii and Enterobacteriaceae). Tigecycline susceptibility of carbapenem-susceptible and carbapenem-non-susceptible isolates was 85.2% and 83.6% (χ2 = 0.15, P > 0.05) using the reference method. In conclusion, in this study MTS showed the best correlation with BMD for tigecycline MICs. Based on in vitro testing, tigecycline can be considered an equally useful choice for infections caused by carbapenem-susceptible and carbapenem-non-susceptible isolates.