Silent Colistin Resistance by VITEK 2 Compact Versus Broth Microdilution in Bloodstream Klebsiella pneumoniae and Escherichia coli: A Prospective Single-Centre Study

Background/Objectives: Colistin remains a last-resort therapy for carbapenem-resistant Klebsiella pneumoniae bloodstream infections, yet its accurate reporting depends on the reference broth microdilution (BMD) method that few clinical laboratories routinely deploy. We aimed to determine the prevalence of colistin resistance in bloodstream isolates and to quantify the diagnostic performance of the widely used VITEK 2 Compact automated system in a Turkish tertiary-care hospital.

Methods

We prospectively analysed 253 consecutive non-duplicate blood culture isolates (144 K. pneumoniae; 109 Escherichia coli) obtained at a Turkish tertiary-care hospital between July 2022 and March 2023. Paired colistin susceptibility testing used BMD (EUCAST guidance, polypropylene plates, no surfactant) and VITEK 2 Compact with AST-N326 cards. EUCAST v12.0 breakpoints (susceptible ≤ 2 mg/L; resistant > 2 mg/L) were applied. Very major error (vME) rate was benchmarked against the ISO 20776-2 acceptance threshold of 3%.