MIC analysis and provisional epidemiological cut-off values (COWT) determination

From the distribution of MICs values obtained, MIC₅₀, MIC₉₀, and CO_WT were calculated. The abbreviation CO_WT will be used to refer to these results as the values are proposals based on this isolate collection. The abbreviations ECV and ECOFF will not be used as they refer to consensus-based epidemiological cut-off values from CLSI and EUCAST, respectively.

Provisional CO_WT values were statistically determined according to two methods, one proposed by Turnidge et al. and second one by Kronvall (Turnidge et al., 2006; Kronvall, 2010) which will be referred to later on as “Turnidge method” or “Kronvall method.” Fully automated and freely available Excel spreadsheet calculators to apply the normalized resistance interpretation (NRI) method (Kronvall, 2010) [available at http://www.bioscand.se/nri/ used with permission from the patent holder, Bioscand AB, TÄBY, Sweden (European patent No 1383913, US Patent No. 7,465,559)] and ECOFFinder MS (available at http://clsi.org/standards/micro/ecoffinder/) were used. Following their author's recommendation, CO_WT were computed for 97.7 and 99% of the population level inclusion in the wild type population, respectively. Numbers and percentages of non-wild type isolates were calculated afterwards.

Calculations were performed for each antimicrobial, at genus level on the whole dataset and at species level, when at least 30 isolates from the same species were encountered in the collection (CLSI Report cited by Smith et al., 2013).