Background: The increasing global concern of colistin resistance, especially among multidrug-resistant bacteria, highlights the urgent need for enhanced surveillance. This study compared the VITEK 2 compact automated system with in-house broth microdilution (BMD) for Gram-negative isolates, assessing agreement and errors.
Materials and Methods: In this retrospective comparative analytic study conducted from March 1, 2023, to August 2023, a total of 879 Gram-negative isolates, including Enterobacterales, Acinetobacter baumannii complex, and Pseudomonas aeruginosa (P.A), were identified, tested, and analyzed according to CLSI M-100 Ed-33 and EUCAST version 13.1, 2023 guidelines for colistin susceptibility testing using the VITEK 2 compact automated system with reference to the in-house BMD method in a microbiology laboratory at a tertiary care hospital.
Results: Out of 879 isolates, colistin resistance rates were 3.75% (BMD) and 4.32% (VITEK 2). VITEK 2 missed resistance in 2 isolates, per EUCAST guidelines. Comparing VITEK 2 with in-house BMD for colistin susceptibility in Enterobacterales: the overall essential agreement (EA) was 83.4%, and the categorical agreement (CA) was 99.4%. The very major error (VME) rate was 5.88%, and the minor error (ME) rate was 0.41%. For Pseudomonas aeruginosa, the overall EA was 71.5%, with a CA of 97%. The VME rate was 1%, and the ME rate was 2.68%. Regarding the Acinetobacter baumannii complex, EA was 76.53%, CA 100%, with no ME or VME as per the guidelines.
Conclusion: Discrepancies in colistin susceptibility testing were noted. BMD should confirm VITEK 2 results for both resistant and susceptible isolates.
Keywords: Broth microdilution; Colistin susceptibility testing.
