Interpretive accuracy of the disk diffusion method for testing newer orally administered cephalosporins against Morganella morganii.

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.