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

Abstract

bIn this study, we introduce a new approach for determination of epidemiologic cutoffs (ECOFFs) and resistant-population cutoffs (RCOFFs) based on receiver operating characteristic (ROC) curves. As an example, the method was applied for determination of ECOFFs for seven different beta-lactam antibiotics and wild-type populations of Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae. In addition, RCOFFs were determined for bacterial populations with defined resistance mechanisms ("resistotypes"), i.e., extended-spectrum beta-lactamase (ESBL)-positive E. coli, ESBL-positive K. pneumoniae, and ESBL-positive E. cloacae; AmpC cephalosporinase-positive E. coli and AmpC-positive K. pneumoniae; and broad-spectrum beta-lactamase (BSBL)-positive E. coli. RCOFFs and ECOFFs are instrumental for a systematic characterization of associations between resistotypes and wild-type populations. In antimicrobial susceptibility testing (AST), clinical isolates are classified as resistant or susceptible on the basis of clinical breakpoints (CBPs), i.e., a threshold inhibition zone diameter size to which the treatment outcome is correlated. Organizations such as the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI) recommend CBPs for a wide range of antibiotic-pathogen combinations (1, 2). CBPs are derived from a combination of pharmacokinetic and pharmacodynamic (PK/PD) data, clinical outcome data, and in vitro MIC data (3, 4). EUCAST has revived the concept of the microbiological breakpoint proposed by Williams (5) and introduced the epidemiological cutoff (ECOFF), which is used as microbiological evidence in the process of CBP setting. EUCAST defined the ECOFF as "the upper MIC value of the wild-type distribution" and as the cutoff which "separates microorganisms without (wild-type) and with acquired resistance mechanisms (non-wild-type) to the agent in question" (1, 3). In an accompanying paper, Valsesia et al. (6) argue that in order to improve the description of antimicrobial susceptibility patterns in bacterial populations, the relationship between wild-type and non-wild-type populations should be considered. A complementary cutoff term is proposed for this purpose: the resistantpopulation cutoff (RCOFF). The RCOFF is defined as the largest inhibition zone diameter (or the lowest MIC) that delineates a non-wild-type population and complements the ECOFF for improved discrimination of populations carrying a resistance mechanism (the "resistotypes") from wild-type bacterial populations.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). While this method can be applied for cutoff estimat...

Emergence of extended-spectrum β-lactamase (ESBL) and/or carbapenemase producing Enterobacteriaceae (CPE) and their antimicrobial resistance

“…In Enterobacter spp. is intrinsically present chromosomal encoded class C particularly inducible β-lactamase (13,24). During the treatment by 3rd generation of cephalosporins (3GS) and hyperproduction (high level cephalosporinase) of socalled derepressed AmpC β-lactamase, the therapy can fail (25).…”

Wild-Type and Non-Wild-Type Mycobacterium tuberculosis MIC Distributions for the Novel Fluoroquinolone Antofloxacin Compared with Those for Ofloxacin, Levofloxacin, and Moxifloxacin

“…The clinical breakpoints (known as critical concentrations [CCs] in the tuberculosis field) employed for this purpose should be based on clinical, pharmacokinetic/pharmacodynamic, and, ideally, clinical outcome data, which, for a variety of reasons, are difficult to obtain for tuberculosis drugs ( 27 ). As a result, an important aim of DST for the majority of tuberculosis drugs is to distinguish wild-type from non-wild-type strains [i.e., strains with elevated MICs compared with those for strains that (i) have never been exposed to the agent or class of agent in question and (ii) are not intrinsically resistant] using the ECOFF, which represents the highest concentration of the wild-type distribution determined by modern microbiological principles pioneered by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) ( 12 , 23 , 27 – 30 ). In other words, the ECOFF represents the lowest possible CC and some non-wild-type strains might remain treatable, as proposed for MFX, albeit on the basis of limited evidence (i.e., the CC of 2 μg/ml set by the World Health Organization [WHO] is higher than the ECOFF) ( 9 , 29 , 31 ).…”

Determination of MIC Distribution and Epidemiological Cutoff Values for Bedaquiline and Delamanid in Mycobacterium tuberculosis Using the MGIT 960 System Equipped with TB eXiST

“…The ECOFF can be estimated by visual inspection of a histographic population analysis of the tested strains (eyeball method) or calculated statistically (18,19). We used visual inspection and a receiver operating characteristic (ROC) curve-based method to determine the ECOFF (20). Drug stability was tested in four series of 11 different drug concentrations in MGIT tubes using M. tuberculosis H37Rv as the test strain.…”