Objective: There is a lack of literature comparing the performance of the BD Phoenix M50 (PHX) with the Vitek 2XL (V2) system in susceptibility testing of multidrug resistant organisms. We evaluated the accuracy, reliability and reproducibility of these systems using a collection of organisms with confirmed resistances (molecular +/- phenotypic).
Methods: The PHX NMIC-404 and PMIC-84 panels and the V2 AST-N246 and AST-P612 cards were tested for analytical reproducibility across different organism categories. 10 ATCC organisms, 200 Gram negative bacilli (GNB) and 100 Gram positive cocci (GPC) were used in this study. Isolates were tested simultaneously on both platforms and minimum inhibitory concentration (MIC) values of antibiotics common to both systems compared. Organisms with >1-fold difference in MIC had broth microdilution (BMD), (Thermo Fisher Sensititre panels GN3FG- and GPALL1FG+) performed as a reference standard.
MIC data was compared across the instruments and analysed by MIC and categorical interpretation (CSLI standards). MIC measurements were classified as concordant (EA – essential agreement) if MIC results were within 1 dilution of each instrument, while categorical interpretation was classified (CA – category agreement) using standard definitions of susceptibility errors (minor (mD), major (MD) and very major discrepancies (VMD).
Results: 10 ATCC strains each tested 5 times found 100% EA on both instruments.
For the 200 GNB tested, overall EA and CA were 95.8% and 90.7%, respectively. 50.5% of isolates required BMD to resolve discrepancies. The VMD, MD and mD rates were 0.16%, 0.48%, 1.18% and 0.29%, 0.61%, 1.09% for PHX and V2, respectively. The antibiotic with highest percentage of discrepant MICs overall was cefepime with an EA and CA of 79% and 65.5%.
Relative discrepancies were low for GPC, the overall EA and CA was 95.0% and 94.7%.
Conclusion: The performance of both systems was comparable with a low level of VMD and MD. Evaluation of a resistant population revealed generally acceptable results similar to more susceptible populations.