Estimating the precision of disc diffusion antibiotic susceptibility data published by the European committee on antimicrobial susceptibility testing

Abstract

Analysis of 163 disc diffusion data sets, 115 for bacterial species groups and 48 for types strains, published by EUCAST, was used to optimize the setting of the parameters of a standardized protocol for normalized resistance interpretation of these data. The standard deviations of the normalized distributions of these data sets, calculated using this standardized protocol, were shown to be independent of the means of their respective distributions. These standard deviations could, therefore, be used as a metric to quantify the precision of disc diffusion data sets. The median value of the standard deviations for 115 EUCAST data sets produced in multiple laboratory studies of bacterial species groups was 2.3 mm and that for the 48 EUCAST data sets produced in multiple laboratory studies of type strains was 1.5 mm. It is argued that this standardized method for estimating the precision of disc diffusion data provides a tool by which individual laboratories can assess the quality of the disc diffusion data they produce.

Effect of incubation temperature and time on the precision of data generated by antibiotic disc diffusion assays

“…Using the standard deviation as a measure of precision (Smith & Kronvall ), these analyses of the multilaboratory data sets demonstrated that reducing the temperature at which assays were performed and increasing the incubation time were associated with reductions in the precision of the data obtained (Tables and ). However, before these observations can be taken as evidence that the reductions of the temperature of incubation and the increases in the incubation time are causally related to reductions in precision of the data sets obtained, two potentially confounding factors need to be considered.…”

“…For investigations of the effects of test conditions, such data sets have the advantage that their precision could not be affected by the taxonomic diversity of the strains studied. However, probably for this reason, the standard deviations of zone data sets obtained from type strain studies have been shown to be significantly lower than those obtained from the analysis of bacterial species groups (Smith & Kronvall ). Thus, the quantitative measures of precision from type or reference strain studies made in this work cannot and should not be applied to the prediction of the precision to be expected from zone data obtained from multiple members of a bacterial species.…”

“…The standard form of NRI analysis (Kronvall ; Smith & Kronvall ) is designed to obviate any possible interference by strains with low levels of resistance on the calculation of the means and standard deviations of the normalized WT distributions. It achieves this by calculating these parameters from the high‐end half of the putative WT zone size distributions.…”

“…Smith & Kronvall () have demonstrated that the standard deviation of the normalized distributions of zones for WT (fully susceptible) strains, calculated by the application of a standardized NRI method, provides a suitable measure of the precision of zone data sets. Smith et al .…”

Synergy of β-Lactams with Vancomycin against Methicillin-Resistant Staphylococcus aureus: Correlation of Disk Diffusion and Checkerboard Methods

“…Many factors affect the zone of inhibition during disk diffusion studies, including variations in the culture media; the preparation and handling of inoculum suspensions, inoculated plates, and antimicrobial disks; the duration of incubation of the plates; and the reading of zones (19). A natural variation of 1 to 2.4 mm was described in previous studies (19,20). Our definition of synergy included a consideration of these natural variations.…”