Comparison of the E test (PDM epsilometer) and broth microdilution susceptibility tests for members of the Bacteroides fragilis group.

The E test, a quantitative disk diffusion susceptibility test, was compared with the National Committee for Clinical Laboratory Standards (NCCLS) standardized broth microdilution (BMD) test. A total of 101 isolates in the Bacteroides fragilis group were tested against five antibiotics: cefoxitin, cefotaxime, clindamycin, imipenem, and piperacillin. An agreement of 85%-87% was observed between the E and BMD tests for cefotaxime and clindamycin, 74%-75% for piperacillin and imipenem, and 65% for cefoxitin. Four major (false-resistant) interpretive discrepancies (two each with cefotaxime and piperacillin) and no very major (false-susceptible) discrepancies were observed at 24 hr of incubation. The number of major discrepancies increased to nine when the E test strips were incubated for 48 hr. Although the number of major and very major discrepancies were low for the individual antibiotics, the E test is currently unacceptable with the high proportion of minor discepancies.