The accuracy of four commercial broth microdilution tests in the determination of the minimum inhibitory concentration of colistin

Abstract

Colistin is considered as one of the last-resort antibiotics and reliable antimicrobial susceptibility testing is therefore crucial. The reference standard for AST according to EUCAST and CLSI is broth microdilution (BMD). However, BMD is labor intensive to perform. Commercial antimicrobial susceptibility tests derived from BMD method are available. We investigated the performance of four different commercial tests: Sensititre™, SensiTest™ Colistin, Micronaut MIC Strip Colistin and UMIC Colistin using 70 clinical isolates (half of them was deemed by VITEK2 as resistant), including isolates from cystic fibrosis patients and mcr-1 bearing isolates. We used two reference standards: BMD and composite MIC as determined by all four tests. Sensititre™ had essential agreement (EA, defined as minimum inhibitory concentration within ± 1 dilution) of 87% and 89% compared to BMD and composite reference standard, respectively. For SensiTest™, the EA’s were 93% and 90%. For UMIC, 87% and 90%, and for Micronaut, 83% and 84%. All four tests demonstrated categorical agreement (CA) above 90%. CA for SensiTest™ and Micronaut was both 96%, UMIC 94%, and Sensititre™ 93%. All tests were reproducible as tested in two quality control isolates. In conclusion, in clinical isolates from a large referral center, the four commercial tests for determination of colistin minimum inhibitory concentrations showed acceptable performance.

A Novel Lipid-Based MALDI-TOF Assay for the Rapid Detection of Colistin-Resistant Enterobacter Species

“…However, the limitation of this study is the low specificity, 53.4%. The low specificity of this assay can be explained by the several known challenges with polymyxin AST, especially with Enterobacter species and K. aerogenes ( 40 – 44 ). Broth microdilution is currently the “gold standard” for MIC determination since disc diffusion and Etest have higher false-susceptibility rates due to improper diffusion ( 43 ).…”

Determination of Antimicrobial Resistance Patterns in Salmonella from Commercial Poultry as Influenced by Microbiological Culture and Antimicrobial Susceptibility Testing Methods

“…These differences may have been caused by methodological variations of the two AST methods, including antimicrobial compound preparation (dry powder in the Sensititre ® plate vs. wet solution in the broth breakpoint plate), inoculum preparation (overnight incubation onto blood agar plates vs. in the 96-well plate with MH broth), reaction volume (50 µL in each well of the Sensititre ® plate vs. 200 µL in each well of broth breakpoint ), reaction concentration (1.4 × 10 5 cfu/mL, estimated by McFarland standards vs. 1.5 × 10 6 cfu/mL, estimated by OD 600 reading), and interpretation of AMR results (Vizion TM Digital MIC Viewing System vs. OD600 reading). In early comparative studies, the Sensititre ® platform was compared to other susceptibility tests (MicroScan, Vitek2, E-test, and Micronaut strip) for their essential and categorical agreements with broth micro-dilution method [ 24 , 25 , 26 , 27 ]. The Sensititre ® method exhibited 97.1% essential agreement with the reference broth micro-dilution method in measuring Klebsiella pneumoniae resistance to polymyxins and 92.5% for colistin [ 26 ], and acceptable categorical agreements (>90%) in measuring colistin and polymyxin B resistance in Enterobacteriaceae [ 24 , 27 ].…”

“…In early comparative studies, the Sensititre ® platform was compared to other susceptibility tests (MicroScan, Vitek2, E-test, and Micronaut strip) for their essential and categorical agreements with broth micro-dilution method [ 24 , 25 , 26 , 27 ]. The Sensititre ® method exhibited 97.1% essential agreement with the reference broth micro-dilution method in measuring Klebsiella pneumoniae resistance to polymyxins and 92.5% for colistin [ 26 ], and acceptable categorical agreements (>90%) in measuring colistin and polymyxin B resistance in Enterobacteriaceae [ 24 , 27 ]. No major errors were found with the Sensititre ® system [ 25 ].…”

How to: screening for mcr-mediated resistance to colistin

“…showing the correlation with reference methods; the essential agreement values can slightly differ between individual products [47][48].…”