Quantitative methods that require measurement of zone diameters give the most precise estimate of the susceptibility of bacteria to antimicrobial agents. One such standard procedure1 has been recommended for use with disks to test susceptibility of organisms to ceftizoxime. Organisms should be tested with the ceftizoxime disk, since ceftizoxime has been shown by in vitro tests to be active against certain strains found resistant when other beta-lactam disks are used. Resistant A report of Susceptible indicates that the pathogen is likely to be inhibited by generally achievable blood levels. A report of Resistant indicates that achievable concentrations of the antibiotic are unlikely to be inhibitory and other therapy should be selected.
Standardized procedures require the use of laboratory control organisms.
The 30 mcg ceftizoxime disk should give the following zone diameters. Susceptible organisms produce zones of 20 mm or greater, indicating that the test organism is likely to respond to therapy. Resistant organisms produce zones of 10 mm or less, indicating that other therapy should be selected. As with standard disk diffusion methods, dilution procedures require the use of laboratory control organisms. Standard ceftizoxime powder should give MIC values in the following ranges.