Quality Control for β-Lactam Susceptibility Testing with a Well-Defined Collection of Enterobacteriaceae and Pseudomonas aeruginosa Strains in Spain

Abstract

Eighteen Enterobacteriaceae and Pseudomonas aeruginosa strains, 16 of them with well-defined ␤-lactam re sistance mechanisms, were sent to 52 Spanish microbiology laboratories. Interpretative categories for 8 extended-spectrum ␤-lactams were collected. Participating laboratories used their own routine susceptibility testing procedures (88% automatic systems, 10% disk diffusion, and 2% agar dilution). Control results were established by two independent reference laboratories by applying the NCCLS microdilution method and interpretative criteria. Interpretative discrepancies were observed in 16% of the results (4.4% for cefepime, 3.0% for aztreonam, 2.8% for piperacillin-tazobactam, 1.7% for cefotaxime [CTX] and ceftazidime, 1.1% for ceftriaxone, 0.9% for meropenem, and 0.3% for imipenem). High consistency with reference values (<5% of major plus very major errors) was observed with (i) American Type Culture Collection quality control strains; (ii) strains with low-efficiency mechanisms inactivating extended-spectrum ␤-lactams, such as OXA-1-producing Escherichiacoli or SHV-1-hyperproducing Klebsiella pneumoniae; (iii) strains with highly efficient mechanisms, such as SHV-5 porin-deficient K. pneumoniae, CTX-M-10 in Enterobacter cloacae hyperproducing AmpC, and P. aeruginosa with the MexAB OprM efflux phenotype or hyperproducing AmpC. Low consistency (>30% major plus very major errors) was detected in K1-producing Klebsiella oxytoca, CTX-M-9-producing E. coli, and in OprD ؊ P. aeruginosa strains. Extended-spectrum ␤-lactamase (ESBL)-producing strains accounted for 86% of very major errors. Recognition of the ESBL phenotype was particularly low in Enterobacter cloacae strains (<35%), due to the lack of NCCLS-specific rules in this genus. A K1-producing K. oxytoca was misidentified by 10% of laboratories as an ESBL producer. The use of well-defined resistant strains is useful for improving proficiency in susceptibility testing in clinical laboratories.Antimicrobial susceptibility testing is one of the most important tasks of clinical microbiology laboratories (11). It is performed daily in clinical laboratories by standardized methods. The disk diffusion technique has been extensively used for this objective; however, semiautomated or automated systems based on microdilution techniques are replacing this method, at least for nonfastidious organisms (9).Quality assurance of antimicrobial susceptibility testing is commonly performed by using internal quality control protocols, which monitor the precision and accuracy of the method, the performance of the reagents, and the performance of the microbiologist or technicians carrying out these procedures (14). These protocols often use organisms susceptible to the majority of antimicrobial agents (24) and, to a lesser extent, resistant strains (36). Additional external quality control assessments (proficiency testing) are necessary in the quality assurance of antimicrobial susceptibility testing methods (2). In this case, organisms with susceptibility defined by reference ...

Two-Center Collaborative Evaluation of Performance of the BD Phoenix Automated Microbiology System for Identification and Antimicrobial Susceptibility Testing of Gram-Negative Bacteria

“…This is in contrast with what was reported by Steward et al (29), who observed a high rate of MEs when using other automated systems and suggested laboratories should consider a second independent antimicrobial susceptibility testing method to validate carbapenem intermediateresistant results. On the other hand, our results are in accordance with those reported by R. Cantón et al (4), U. Eigner et al (9), and P. Giakkoupi et al (14). On this matter, Pitout et al have recently recommended that all imipenem-nonsusceptible P. aeruginosa isolates be routinely screened for metallo-␤-lactamase production using the EDTA disk screening test and that confirmation by PCR be performed at a regional reference laboratory (25).…”

“…The level of performance with the ␤-lactams was a little lower, but comparable to those reported for other commercial methods (4,6,7,8,9,16,18,25,29,31). Within the class, the Phoenix system showed excellent performance for the carbapenems, with no VMEs, no MEs, and very few mEs, ranging from 0.5 to 5.2%.…”

Detection of Resistance to Beta-Lactamase Inhibitors in Strains with CTX-M Beta-Lactamases: a Multicenter External Proficiency Study Using a Well-Defined Collection of Escherichia coli Strains

“…Surprisingly, the complex mutant phenotype (CM type) was only suspected in a strain carrying natural CTX-M-14 (CCSRC10) in an OmpF Ϫ background. Previously, CLSI or national breakpoints (19) were followed in Spain, but currently, the EUCAST criteria are increasingly being introduced (25% in our study compared with 14% in a recent published study [29] and 0% in previous ones [30,31]). Globally, the error rates were slightly higher (but not significant) in centers using the CLSI criteria (11.8%) than in centers following the EUCAST criteria (10.3%), except for the case of VMEs (3.0% of discrepancies when using EUCAST criteria compared to 0.7% in centers that used CLSI criteria).…”

Predicting antimicrobial susceptibilities for Escherichia coli and Klebsiella pneumoniae isolates using whole genomic sequence data

“…In 21 instances where the co-amoxiclav genotypic prediction disagreed with the BD Phoenix results (all involving genotypically susceptible isolates that were resistant by BD Phoenix), all isolates were susceptible according to gradient diffusion, suggesting that in 15% of tests automated phenotyping was overcalling resistance. Problems with the correct assessment of susceptibility by phenotyping for β-lactam/β-lactamase inhibitor combinations have been observed previously, 34 particularly in the context of complex β-lactamase genotypes, 35 which were disproportionately represented in our dataset.…”