Evaluation of the VITEK 2 second-generation cassette for colistin susceptibility testing in carbapenem-resistant gram-negative bacteria.

The global rise in antimicrobial resistance has increased morbidity and mortality, particularly among patients with multidrug-resistant Gram-negative infections. Colistin, a last-resort therapeutic, has faced a concerning emergence of resistance. Accurate detection is therefore critical in clinical microbiology laboratories. Although broth microdilution (BMD) is the only reference method recommended for colistin susceptibility testing, its labor-intensive process has driven the search for automated alternatives. The VITEK 2 system is widely used in routine diagnostics, but international guidelines do not recommend automated platforms for colistin testing. A second-generation colistin susceptibility cassette (XN-21) was recently developed to address these limitations. This study evaluated the performance of the VITEK 2 XN-21 cassette against BMD. Identification and susceptibility testing were performed using VITEK 2 GN and AST-420 cards. Colistin susceptibility was assessed by both BMD and XN-21. Isolates classified as susceptible by BMD but resistant by VITEK 2 underwent population analysis profiling (PAP) to investigate heteroresistance. Results were interpreted according to ISO 20776-2 standards. Colistin resistance was detected in 59 % of isolates by BMD and 52 % by XN-21. The sensitivity and specificity of XN-21 were 81.3 % and 90.2 %, respectively. Essential and categorical agreement with BMD were 73 % and 85 %. Very major error and major error rates were 11 % and 4 %. Among 11 very major errors, nine were Klebsiella pneumoniae; PAP confirmed heteroresistance in three of four major error isolates. The VITEK 2 XN-21 cassette offers convenience but does not yet meet reference standards. Its potential in detecting heteroresistant subpopulations merits further evaluation.