Antimicrobial susceptibility tests with cefotaxime and correlation with clinical bacteriologic response

Cefotaxime is a new cephalosporin with a wide spectrum of antimicrobial activity. For broth and agar dilution tests, the MIC breakpoints suggested for defining bacterial susceptibility are less than or equal to 8 microgram/ml for susceptible, 16-32 microgram/ml for moderately susceptible (indeterminate), and greater than 32 microgram/ml for resistant. For disk diffusion tests, the zone diameter breakpoints suggested are greater than or equal to 23 mm for susceptible, 15-22 mm for moderately susceptible, and less than or equal to 14 mm for resistant. Examination of the bacteriologic outcome of infection for 1,440 patients treated with cefotaxime shows that the results of disk diffusion tests correlated well with clinical response. The organisms that had cefotaxime disk zones of greater than or equal to 23 mm were eradicated in 89% of cases. Clinical response of urinary tract infections due to Pseudomonas aeruginosa was unrelated to disk results for bacterial isolates; the rate of bacteriologic cure was 55%-64% regardless of category. It is further recommended that cefotaxime and other cephalosporins not be tested against the enterococci.