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Journal of Clinical Microbiology, May 2003, p. 1912-1918, Vol. 41, No. 5
0095-1137/03/$08.00+0     DOI: 10.1128/JCM.41.5.1912-1918.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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

Rafael Cantón,1* Elena Loza,1 María Del Carmen Conejo,2 Fernando Baquero,1 Luis Martínez-Martínez,2,3 and MENSURA Collaborative Group1,2,3,{dagger}

Servicio de Microbiología, Hospital Universitario Ramón y Cajal, 28034 Madrid,1 Departamento de Microbiología, Facultad de Medicina, Universidad de Sevilla,2 Servicio de Microbiología, Hospital Universitario Virgen Macarena, 41009 Seville, Spain3

Received 18 September 2002/ Returned for modification 3 November 2002/ Accepted 27 January 2003

Eighteen Enterobacteriaceae and Pseudomonas aeruginosa strains, 16 of them with well-defined ß-lactam re sistance mechanisms, were sent to 52 Spanish microbiology laboratories. Interpretative categories for 8 extended-spectrum ß-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 ß-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 ß-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.

* Corresponding author. Mailing address: Hospital Universitario Ramón y Cajal, Carretera de Colmenar, Km 9.1, 28034 Madrid, Spain. Phone: 34-913368330. Fax: 34-913368809. E-mail: rcanton.hrc.{at}salud.madrid.org.

{dagger} Participants in the MENSURA Collaborative Study Group are listed in Acknowledgements.

Journal of Clinical Microbiology, May 2003, p. 1912-1918, Vol. 41, No. 5
0095-1137/03/$08.00+0     DOI: 10.1128/JCM.41.5.1912-1918.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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