Genus-specific Agreement, Error Rates, and Correlation between Broth Microdilution Test and Vitek 2 Compact System with Regard to Colistin Susceptibility Testing in Clinical Isolates of Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter Species
Citation Information :
Chouhan K, Bhattacharya S. Genus-specific Agreement, Error Rates, and Correlation between Broth Microdilution Test and Vitek 2 Compact System with Regard to Colistin Susceptibility Testing in Clinical Isolates of Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter Species. J Acad Clin Microbiol 2024; 26 (2):35-39.
Aims and background: Broth microdilution (BMD) test is the preferred method for colistin susceptibility testing. However, Vitek (bioMérieux, USA) is commonly used because of user ease and cost considerations. Although previous studies have shown different agreement and error rates of the Vitek in comparison to BMD, there is a dearth of data regarding genus- and species-specific differences in error and agreement rates. This is important since such data may be the starting point of future research.
Materials and methods: In this observational study, 705 isolates were studied [Klebsiella pneumoniae (n = 254), Escherichia coli (n = 275), Enterobacter cloacae (n = 20), Citrobacter spp. (n = 10), Acinetobacter spp. (n = 51), Pseudomonas aeruginosa (n = 92)]. Cluster- and convenience-based sampling methods were used. The study was conducted between November 2022 and February 2023. The BMD test was performed using Thermo Scientific reagents and the Sensititre Vizion Digital minimum inhibitory concentration (MIC) Viewing System. Antibiotic susceptibility testing (AST) by the Vitek-2 Compact system was performed using reagent AST-N281 cards.
Results: Overall, 93.9% and 94.75% of isolates were intermediate to colistin by BMD and Vitek, respectively. Essential agreement (EA) and categorical agreement (CA) of Vitek and BMD were 88.09 and 97.73%, respectively. Very major error (VME) of Vitek was 23.81% (10/42), while major error (ME) was 0.76% (5/662). A high correlation (r > 0.7) in MIC values was observed between BMD and Vitek for Klebsiella pneumoniae and Citrobacter spp., while a moderate correlation (0.3 < r < 0.7) was observed for other species (Pearson correlation).
Conclusion: This is one of the few studies from India that demonstrates the error rates of Vitek for a specific genus (e.g., Klebsiella spp., VME: 7.69%) to be significantly different from those of all organisms combined (VME: 23.81%). Although the overall EA of Vitek was 88.23%, the EA for Klebsiella spp. was only 73.93%. Genus- and species-dependent differences were observed with regard to MIC correlations between the two methods.
Clinical significance: While testing, reporting, and interpreting colistin susceptibility for clinical management actions, it is important to understand the specific limitations of susceptibility test systems.
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