Reliability of Vitek 2 (Automated) Versus Reference Broth Microdilution Method (Manual) in Colistin Susceptibility Assessment of MDR Gram Negative Bacilli: Insights from a Tertiary Care Hospital in Northern India

Background: Colistin serves as a critical therapeutic option for infections caused by multidrug-resistant Gram-negative bacteria. Due to its clinical importance, accurate and timely determination of colistin susceptibility is essential. While broth microdilution (BMD) remains the gold standard recommended by EUCAST and CLSI, automated systems such as VITEK®2 are often employed for convenience in clinical microbiology laboratories. Aim and Objective:This study aimed to assess the accuracy and performance of colistin minimum inhibitory concentration (MIC) determination by VITEK®2 compared to the reference BMD method in Gram-negative isolates from an intensive care unit (ICU) in a tertiary care hospital. Material and Methods: Gram-negative clinical isolates from ICU patients were analyzed from February 2025 to July 2025. Colistin MICs were determined using both BMD and VITEK®2. Comparative analysis included parameters such as very major error (VME), major error (ME), essential agreement (EA), categorical agreement (CA), sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Results: Out of 304 isolates, the majority were recovered from urine (42%), followed by sputum samples (32%), endotracheal aspirates (8%), pus ( 6.9%), blood ( 6.5%), and bronchoalveolar lavage (4%). The predominant species included Klebsiella pneumoniae (34.2%),E. coli( 21.7%), Enterobacter cloacae ( 4.6%),Acinetobacter baumannii (22%), and Pseudomonas aeruginosa (17.1%). VITEK®2 demonstrated high sensitivity (>97%) but poor specificity (45.45%). The PPV and NPV were 90.39% and 92.85% respectively. VME rates ranged between 54.5–100%, with E. coli showing the highest (100%). ME was 74.9%. The overall EA was 62.10%, and CA was 92.10%. Conclusion: VITEK®2 failed to accurately detect colistin resistance in 63% of the resistant isolates. The observed VME and ME rates exceeded acceptable thresholds, and EA did not meet the ≥90% benchmark. Despite its high sensitivity, the low specificity of VITEK®2 limits its reliability for colistin susceptibility testing. BMD should remain the preferred method for determining colistin MICspecially in critical care settings.