Evaluation of MicroScan and VITEK 2 systems for susceptibility testing of Enterobacterales with updated breakpoints

We compared the performance of two commercial antimicrobial susceptibility testing (AST) systems for contemporary Enterobacterales strains using broth microdilution (BMD) as the reference standard with 2022 Clinical and Laboratory Standards Institute or current FDA breakpoints applied. Enterobacterales clinical isolates with BMD results were tested in parallel using VITEK 2 AST test cards (N802, XN15, bioMérieux, Hazelwood, MO, USA) and MicroScan NM56 AST panels (Beckman Coulter, Sacramento, CA, USA). The 200 isolates (57 Escherichia coli, 55 Klebsiella pneumoniae, 21 Enterobacter cloacae complex, 18 Serratia marcescens, 12 Proteus mirabilis, 10 Citrobacter koseri, 9 Citrobacter freundii, 8 Klebsiella oxytoca, 5 Klebsiella aerogenes, 3 Providencia stuartii, and 2 Morganella morganii) included 25% extended-spectrum beta-lactamase (ESBL), 23% carbapenem-resistant Enterobacterales (CRE), and 4.5% AmpC resistance phenotypes. For the 28 antimicrobial agents tested, essential agreement (EA, MIC within ±1 doubling dilution), categorical agreement (CA, same categorical interpretation: susceptible, intermediate, susceptible dose dependent, and resistant), and error rates were calculated using BMD as the reference standard. Time to results (TTR) was determined for each instrument for all 200 isolates. Hands-on time was assessed by timing two technologists each setting up six batches of five isolates on each system. Accuracy was similar between systems with an overall CA > 94% and EA ≥ 96%. The CA was ≥90% for most agents tested on both systems (exceptions were ampicillin-sulbactam, cefoxitin, minocycline, and nitrofurantoin). The MicroScan with Prompt inoculum preparation required less hands-on setup time than VITEK (1.29 vs 1.83 min/isolate). The median instrument TTR was less for VITEK (11.7 vs 18 hours, P < 0.001), yielding an overall faster turnaround time.IMPORTANCEThere are limited data directly comparing the performance of commercial antimicrobial susceptibility testing systems for contemporary bacterial strains using the Clinical and Laboratory Standards Institute broth microdilution method as the reference standard and applying updated breakpoints. These data will hopefully encourage labs to perform the necessary verification or validation studies needed to implement current breakpoints and ensure antimicrobial resistance is detected.