Epidemiological MIC cut-off values for tigecycline calculated from Etest MIC values using normalized resistance interpretation.
OBJECTIVE
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.
Kronvall
Göran
G
Clinical Microbiology--MTC, Karolinska Institute, Karolinska Hospital L2:02, 17176 Stockholm, Sweden. goran.kronvall@ki.se
Karlsson
Inga
I
Walder
Mats
M
Sörberg
Mikael
M
Nilsson
Lennart E
LE
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D004812:Epidemiologic Methods; D006090:Gram-Negative Bacteria / Q000187:drug effects; D006094:Gram-Positive Bacteria / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D008954:Models, Biological; D000078304:Tigecycline
D000900:Anti-Bacterial Agents; D000078304:Tigecycline; D008911:Minocycline