Ceftizoxime and cefoxitin susceptibility testing against anaerobic bacteria: comparison of results from three NCCLS methods and quality control recommendations for the reference agar dilution procedure.

Ceftizoxime and cefoxitin (control) were tested against 99 anaerobic bacteria using three commonly used susceptibility testing methods. The cefoxitin MIC results with the National Committee for Clinical Laboratory Standards (NCCLS) reference agar dilution (RAD) method and the broth microdilution (BMD) method did not differ by more than one log2 dilution step. However, the ceftizoxime BMD MIC results were 2- to 4-fold lower than those produced by the RAD procedure. Broth disk elution (BDE) tests for both cephalosporins (ceftizoxime and cefoxitin) had very high rates of false-susceptible and false-resistant error when compared to the RAD MICs: the BDE tests are not recommended for either cephalosporin. Findings indicate that because of method and possible medium or technical variability, ceftizoxime MICs should be determined only by the NCCLS RAD method. Also, cefoxitin susceptibility should be assessed primarily by RAD or BMD procedures. This should minimize confusion related to ceftizoxime's spectrum and activity against anaerobic bacteria compared to similar beta-lactam drugs. Ceftizoxime quality control guidelines were established for Bacteroides fragilis (ATCC 25285): mode, 64 micrograms/ml, range 32-128 micrograms/ml. Other control strains were not reliable.