Journal Article

Epidemiological MIC cut-off values for tigecycline calculated from Etest MIC values using normalized resistance interpretation

Göran Kronvall, Inga Karlsson, Mats Walder, Mikael Sörberg and Lennart E. Nilsson

in Journal of Antimicrobial Chemotherapy

Published on behalf of British Society for Antimicrobial Chemotherapy

Volume 57, issue 3, pages 498-505
Published in print March 2006 | ISSN: 0305-7453
Published online January 2006 | e-ISSN: 1460-2091 | DOI: http://dx.doi.org/10.1093/jac/dki489
Epidemiological MIC cut-off values for tigecycline calculated from Etest MIC values using normalized resistance interpretation

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  • Medical Oncology
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Objectives: To apply the normalized resistance interpretation (NRI) method to Etest MIC results which have higher precision than conventional log2 dilution MIC tests due to the inclusion of intermediate values. If successful, NRI might provide an objective tool for the definition of epidemiological MIC cut-off values.

Methods: MICs of tigecycline and other antimicrobial agents were determined for 4771 clinical isolates comprising five Gram-positive and 13 Gram-negative species or species groups using the Etest. Histograms of MIC values were constructed for each species and NRI calculations were applied to them. An upper MIC limit of 2.5 SD above the theoretical mean of the normalized distribution was used for setting the epidemiological cut-off values.

Results: Calculated cut-off values for wild-type strains were between 0.11 and 0.96 mg/L for Gram-positive species, and between 0.44 and 8.3 mg/L for Gram-negative species, except for Pseudomonas aeruginosa, which had a cut-off value of 450 mg/L, consistent with earlier reports on the lack of activity of tigecycline against this species.

Conclusions: NRI offers an objective method for the analysis of MICs produced using Etests and the determination of epidemiological MIC cut-off values.

Keywords: antimicrobial susceptibility tests; MIC breakpoints; drug resistance; clinical microbiology; wild-type MIC distributions

Journal Article.  4325 words.  Illustrated.

Subjects: Medical Oncology ; Critical Care

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