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Epidemiological MIC cut-off values for tigecycline calculated from Etest MIC values using normalized resistance
Göran Kronvall1, Inga Karlsson, Mats Walder
1Clinical Microbiology--MTC, Karolinska Institute, Karolinska Hospital L2:02, 17176 Stockholm, Sweden. goran.kronvall@ki.se
The Journal of Antimicrobial Chemotherapy
|January 18, 2006
Summary
The normalized resistance interpretation (NRI) method objectively analyzes Etest MIC data, establishing precise epidemiological cut-off values for antimicrobial resistance surveillance. This method enhances accuracy over traditional tests.
Area of Science:
- Microbiology
- Clinical Microbiology
- Pharmacology
Background:
- Accurate determination of antimicrobial resistance is crucial for effective treatment and public health.
- Conventional Minimum Inhibitory Concentration (MIC) tests have limitations in precision.
- The Normalized Resistance Interpretation (NRI) method offers potential for improved MIC analysis.
Purpose of the Study:
- To evaluate the Normalized Resistance Interpretation (NRI) method using Etest MIC results.
- To assess NRI's capability in defining objective epidemiological MIC cut-off values.
- To compare NRI's precision with conventional log2 dilution MIC tests.
Main Methods:
- Determined MICs for tigecycline and other agents against 4771 clinical isolates using Etest.
- Constructed MIC value histograms for each species.
- Applied NRI calculations, using an upper limit of 2.5 SD above the theoretical mean for cut-off values.
Main Results:
- Calculated wild-type cut-off values ranged from 0.11 to 0.96 mg/L for Gram-positive species.
- Cut-off values for Gram-negative species ranged from 0.44 to 8.3 mg/L.
- Pseudomonas aeruginosa showed a high cut-off value (450 mg/L), indicating tigecycline ineffectiveness.
Conclusions:
- The Normalized Resistance Interpretation (NRI) method provides an objective approach for analyzing Etest MIC data.
- NRI facilitates the accurate determination of epidemiological MIC cut-off values.
- This method enhances the precision of antimicrobial resistance interpretation.