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Interpretive accuracy of the disk diffusion method for testing newer orally administered cephalosporins against
D J Biedenbach1, R N Jones, M E Erwin
1Department of Pathology, University of Iowa College of Medicine, Iowa City 52242.
Abstract:
Eight newer orally administered cephems (cefdinir, cefetamet, cefixime, cefpodoxime, cefprozil, ceftibuten, cefuroxime, and loracarbef) were tested against 100 clinical strains of Morganella morganii to determine the extent of serious interpretive very major (false-susceptible) errors when current criteria for the disk diffusion test are applied. Agar dilution MICs and disk diffusion tests were performed as recommended by the National Committee for Clinical Laboratory Standards (Villanova, Pa.) (NCCLS), and the methods were compared by regression analysis using the method of least squares and by error rate bounding. The following results are listed in the order of increasing error rates: cefdinir, loracarbef, and cefprozil, < or = 1% very major error; ceftibuten, 8% minor errors; cefuroxime, 21% minor errors; cefixime, cefpodoxime, and cefetamet, very major errors of 15, 24, and 36%, respectively. M. morganii produces unacceptable rates of test error with cefuroxime, cefixime, cefpodoxime, and cefetamet. The latter two cephalosporins currently have NCCLS table footnote warnings covering the problem observed with this organism. The inclusion of cefuroxime and cefixime in the NCCLS table footnote is strongly recommended.
Insights
Disk diffusion tests for newer oral cephalosporins against Morganella morganii showed high error rates for cefuroxime, cefixime, cefpodoxime, and cefetamet. Updated interpretive criteria are needed to prevent false susceptibility results.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
Background:
- Oral cephalosporins are widely used for bacterial infections.
- Accurate antimicrobial susceptibility testing is crucial for effective treatment.
- Morganella morganii can exhibit resistance to certain antibiotics.
Purpose of the Study:
- To evaluate the accuracy of disk diffusion testing for newer oral cephalosporins against Morganella morganii.
- To identify potential interpretive errors, specifically very major (false-susceptible) errors.
- To recommend updates to antimicrobial susceptibility testing guidelines.
Main Methods:
- Testing of eight oral cephalosporins against 100 clinical strains of Morganella morganii.
- Comparison of agar dilution Minimum Inhibitory Concentrations (MICs) with disk diffusion test results.
- Analysis using regression and error rate bounding methods.
Main Results:
- Cefdinir, loracarbef, and cefprozil showed very low error rates (<1%).
- Ceftibuten and cefuroxime had minor errors (8% and 21%, respectively).
- Cefixime, cefpodoxime, and cefetamet demonstrated unacceptable very major error rates (15%, 24%, and 36%).
Conclusions:
- Morganella morganii produces unacceptable test error rates with cefuroxime, cefixime, cefpodoxime, and cefetamet using current disk diffusion criteria.
- NCCLS (National Committee for Clinical Laboratory Standards) footnote warnings are already in place for cefpodoxime and cefetamet.
- Inclusion of cefuroxime and cefixime in NCCLS table footnote warnings is strongly recommended to improve accuracy.