Normalized Resistance Interpretation as a Tool for Establishing Epidemiological MIC Susceptibility Breakpoints

Abstract

Normalized resistance interpretation (NRI) utilizes the fact that the wild-type population on the sensitive side is not affected by resistance development, and therefore a normalized reconstruction of the peak can be performed Traditional concepts for setting interpretive breakpoints in antimicrobial susceptibility testing were challenged by J. D. Williams in 1990 when he proposed the setting of so-called microbiological guidelines in relation to the distribution of inhibition zone diameter values (or MIC values) of the susceptible population, for instance two standard deviations (SD) below the mode inhibition zone size (24). This would require a species-related aspect which was not yet accepted, although some reports on this issue had been published (2,12,14). Earlier, O'Brien had shown that an adjustment of interpretive breakpoints in relation to the susceptible peak in zone diameter histograms made results from different laboratories comparable and thereby foresaw later developments (17). In recent years, the term epidemiological breakpoints or epidemiological cutoff (ECOFF), has been proposed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) to describe the upper MIC limit of the susceptible peak in an MIC distribution, the wild-type (WT) population. MIC distributions of WT isolates species-wise for many antimicrobials have been published on the internet by EUCAST and are publicly available (http://www.eucast.org/mic_distributions/) (5, 6).Normalized resistance interpretation (NRI) is an objective method to define the WT population in inhibition zone diameter histograms (4,8,10). The method has been used for setting interpretive breakpoints in an 18-year resistance surveillance, and also in an investigation of meropenem susceptibility in Pseudomonas aeruginosa (3,22). It was recently used extensively in marine microbiology by P. . NRI was also used successfully to analyze Etest MIC data to set epidemiological breakpoints for tigecycline (11). However, a direct application of NRI to regular 2-fold dilution MIC results has not been possible because the mathematical calculations will be based on too few data points. The use of NRI has been extended in the present investigations to regular MIC values by the introduction of helper variables, and NRI has been evaluated as a method for the objective estimation of epidemiological breakpoints using the EUCAST MIC distributions as the material and as the reference (http://www.eucast .org/mic_distributions/). MATERIALS AND METHODSMIC distributions. The publicly available EUCAST MIC distributions include a large number of bacterial species and antimicrobials (http://www.eucast.org /mic_distributions/). For the purpose of testing the NRI method for MIC distributions in the present studies, results for Staphylococcus aureus and Escherichia coli were used. EUCAST claim on their website that the "Histograms display WT organisms." It is not clear, however, which method is used to ensure this selection in the published materials, which can include as ...

Comparison of Three Statistical Methods for Establishing Tentative Wild-Type Population and Epidemiological Cutoff Values for Echinocandins, Amphotericin B, Flucytosine, and Six Candida Species as Determined by the Colorimetric Sensititre YeastOne Method

“…The results obtained, in general, were the same as those found with all 2,119 isolates or fell within one 2-fold dilution, which is in agreement with the work of Turnidge et al and Kronvall, who reported that their methods are also valid for data sets generated by a single laboratory and method (12,27). It is difficult to determine which statistical method defines the ECV best, due to the scarcity of resistant isolates.…”

“…Since the ECVs estimated by the clustering method are on a continuous scale, values were rounded to the nearest higher dilution after reconversion to concentration units. 50 or modal MIC (6,12,27). We used the clustering statistical method to determine ECVs and compared them with those obtained by other methods and with those reported for the CLSI methodology.…”

“…Recently, Pfaller et al used the CLSI methodology to study MIC distributions and define the epidemiological cutoff values (ECVs) of echinocandins for 11 Candida species, including the most common and some of the more uncommon species, as well as the ECVs of AMB and flucytosine (FC) (18,20,23 (12,14,27). Of these three proposed methods, that of Turnidge et al is the most commonly used one.…”

Setting interpretive breakpoints for antimicrobial susceptibility testing using disk diffusion

“…With the intermediate MIC values of the Etest, enough data points were obtained for successful mathematical calculations [65]. Also, regular 2-log dilution MIC distributions have now been analysed using NRI, made possible by the introduction of helper variables [66]. With such objective methods also applied to MIC distributions and the advent of pharmacokinetic/pharmacodynamic-Monte Carlo simulations for defining MIC epidemiological breakpoints, it seems that this changing pattern has come full circle.…”

A Statistical Approach for Determination of Disk Diffusion-Based Cutoff Values for Systematic Characterization of Wild-Type and Non-Wild-Type Bacterial Populations in Antimicrobial Susceptibility Testing

“…To date, several methods have been proposed to determine inhibition zone diameter cutoffs, such as the error rate-bounded method (7), a modified error rate-bounded method (8), the normalized resistance interpretation method (9,10), and few statistical inference approaches (11,12). However, visual inspection of distributions (eyeball method) has usually been applied to determine cutoffs (12)(13)(14)(15)(16).…”