Comparative Evaluation of MicroScan Walkaway 96 Plus ID/AST System and Mikrolatest Broth Microdilution Kit in Assessing In vitro Colistin Susceptibility of Carbapenem-Resistant Clinical Gram-negative Bacterial Isolates: Experience from a Tertiary Care Teaching Hospital in Rishikesh, Uttarakhand

Context: As reports on colistin resistance are slowly emerging from different parts of the world, it is imperative that the clinical microbiology laboratories should generate accurate in vitro colistin susceptibility results. Aim: The aim is to generate preliminary data on the diagnostic utility of MicroScan WalkAway 96 Plus Identification ID/Antimicrobial susceptibility testing AST system in determining in vitro colistin susceptibility of carbapenem-resistant clinical Gram-negative bacterial isolates. Settings and Design: A pilot study was conducted in a tertiary care teaching hospital located in Rishikesh, Uttarakhand, between May and June 2019. Materials and Methods: Thirty-four carbapenem-resistant Escherichia coli, Pseudomonas aeruginosa and Acinetobacter spp. isolated from various non-repetitive clinical samples during the study period, were subjected to antibiotic susceptibility testing using MicroScan ID/AST system. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry was used to confirm identity of these isolates. Additional colistin susceptibility testing of all test isolates was performed using Mikrolatest minimum inhibitory concentration antibiotic susceptibility testing kit (reference method), which is based on broth micro dilution (BMD) principle. Statistical Analysis Used: Fisher’s exact test. Results: 11.8% (4/34) of the test isolates (100% [2/2] Acinetobacter junii, 10% [1/10] E. coli and 14.3% [1/7] P. aeruginosa respectively) exhibited in vitro colistin resistance by BMD method. Categorical agreement between MicroScan ID/AST system and Mikrolatest kit w. r. t in vitro colistin susceptibility test results was as follows: 71.4% (Acinetobacter baumannii), 85.7% (P. aeruginosa) and 100% (A. junii, A. johnsonii, E. coli and Klebsiella pneumoniae), respectively. Two major errors (MEs) for A. baumannii and one very ME for P. aeruginosa respectively were observed. Conclusions: Data generated by this study will be of help to the clinicians who are often faced with the dilemma of treating multi drug resistant infections with limited treatment options.