Checkerboard Insights: Diagnostic Integration of Synergy Testing for Multidrug-Resistant Klebsiella Isolates in Critical Care

Introduction: The rise of multidrug-resistant Klebsiella pneumoniae poses a critical challenge in intensive care units (ICUs), rendering standard monotherapies ineffective and necessitating the exploration of synergistic combination regimens. Methods: This cross-sectional, laboratory-based study was conducted over one year (April 2024 to March 2025) at a tertiary care center in Ghaziabad, India. Initial antimicrobial susceptibility testing of 196 ICU isolates was performed using the VITEK® 2 Compact system (bioMérieux India Private Limited, New Delhi, India). A subset of 100 isolates nonsusceptible to colistin and resistant to companion agents was selected for checkerboard synergy testing using the HiMIC™ (HiMedia Laboratories Pvt. Ltd., Mumbai, India) microdilution method to evaluate combinations of colistin with meropenem, amikacin, and amoxicillin-clavulanate. This study aimed to evaluate the demographic and clinical specimen profiles of ICU patients with MDR/PDR Klebsiella infections and to determine the in vitro synergistic potential of colistin-based combinations using the checkerboard assay. Results: Among the 196 MDR isolates, the largest demographic group comprised patients aged ≥60 years (38.3%), and male patients represented 59.7% of cases. Respiratory specimens were the primary source of isolates (40.3%), followed by blood (19.9%) and pus (14.8%). High resistance rates were observed for amoxicillin-clavulanate (88.3%), meropenem (81.6%), and amikacin (75.5%). In checkerboard testing of 100 selected isolates, the colistin-meropenem combination achieved the highest synergistic activity (12.0%) with zero antagonism. Colistin-amikacin demonstrated 10.0% synergy and 6.0% antagonism, while colistin-amoxicillin/clavulanate showed the lowest synergy (5.0%) and highest antagonism (12.0%). Conclusion: The colistin-meropenem combination exhibited the highest in vitro synergistic activity against MDR Klebsiella isolates, with no antagonistic behavior, highlighting its potential clinical utility in critical care settings.