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

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Abstract
Eighteen Enterobacteriaceae and Pseudomonas aeruginosa strains, 16 of them with well-defined β-lactam re sistance mechanisms, were sent to 52 Spanish microbiology laboratories. Interpretative categories for 8 extended-spectrum β-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 (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 β-lactamase (ESBL)-producing strains accounted for 86% of very major errors. Recognition of the ESBL phenotype was particularly low in Enterobacter cloacae strains (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.

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