Validation of novel cost-effective antimicrobial gradient strips for the determination of carbapenem susceptibility in Enterobacteriaceae.

Objectives: The aim of this study was to validate the use of LML antimicrobial gradient strips for quantitative determination of carbapenem susceptibility in Enterobacteriaceae.

Methods: A total of 95 non-redundant Enterobacteriaceae strains isolated during 2012-2014 were used for this validation study. Initially, LML antimicrobial gradient strips were validated for their performance in comparison with the agar dilution method. The test strip was then validated in comparison with broth microdilution (BMD) and Etest with 24 selected strains using the same inocula and other laboratory parameters.

Results: The LML strip showed 83%, 68% and 86% essential agreement (within ±1 log dilution) with the reference methods of agar dilution, BMD and Etest, respectively; furthermore, essential agreement was >90% within ±2 log dilution. Categorical agreement was ≥87% with all reference methods according to Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. However, the meropenem strip requires performance improvements to fulfil US Food and Drug Administration (FDA) and International Organization for Standardization (ISO) requirements for an antimicrobial susceptibility test device.

Conclusions: In LML antimicrobial gradient strip minimum inhibitory concentrations (MICs) were comparable with Etest MICs and it might serve as a reasonable, cost-effective alternative to Etest for quantitative determination of carbapenem susceptibility in Enterobacteriaceae.