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%. Results: BMD identified colistin resistance in 22.5% (95% CI 17.8–28.1) of isolates, with a striking inter-species disparity: 38.2% for K. pneumoniae versus 1.8% for E. coli (Fisher exact p = 1.1 × 10−13; OR 33.1, 95% CI 8.0–138). VITEK 2 misclassified 32 of 57 truly resistant isolates as susceptible; under the ISO 20776-2 denominator the very major error rate was 56.1% (32/57; 95% CI 43.3–68.3), approximately 18-fold above the ISO acceptance threshold of 3% (p < 0.0001). Overall sensitivity for colistin resistance detection was 43.9% (25/57); for K. pneumoniae alone, sensitivity was 45.5% (25/55) and specificity 95.5% (85/89), and every skipped-well BMD isolate (n = 11) was reported susceptible by VITEK 2. Conclusions: Automated colistin testing underdetects resistant K. pneumoniae bloodstream isolates in a high-prevalence setting. Our findings support continued adherence to BMD as the reference standard for colistin susceptibility in such settings, with automated systems used for screening and complemented by reference-method confirmation of critical isolates.