Comparison of the E-test and conventional agar dilution methods for susceptibility testing of gram-negative anaerobic rods.

The E test was used to determine the susceptibility of 209 anaerobic Gram-negative rods to ampicillin, amoxicillin-clavulanate, ticarcillin, ticarcillin-clavulanate, piperacillin, piperacillin-tazobactam, clindamycin, chloramphenicol, and cefoxitin. Minimum inhibitory (MICs) were read where growth intersected the strips or where the upper indentation intersected the strips for beta-lactamase inhibitor-containing strips. Reference MICs were performed by the National Committee for Clinical Laboratory Standards agar dilution method. Organisms tested consisted of 117 Bacteroides fragilis group, 24 fusobacteria, and 68 other species (mainly Prevotella species). Agreement of MICs by both methods occurred in 33.4% of cases; 80.8% were within one, 99.0% were within two, and 99.95% were within three doubling dilutions apart. E-test results, expressed as MIC50/MIC90 values (microgram/ml), were as follows: ampicillin: 16/> 256; amoxicillin-clavulanate (2:1), 0.5/4; ticarcillin, 16/> 256; tircarcillin-clavulanate (2 micrograms/ml), 0.5/4; piperacillin, 16/> 256; piperacillin-tazobactam (4 micrograms/ml), 1/8; piperacillin-tazobactam (8:1), 2/8; cefoxitin, 8/64; clindamycin, 0.25/4; and chloramphenicol, 2/8. Conventional MIC methodology yielded MIC50s and MIC90s either identical to, or within one doubling dilution of, E-test results. Percentages of strains susceptible varied by < 3% between the two methods, except for cefoxitin (E test 5.2% lower). Based on these results, the E test is an accurate and practical method for use with the agents and organisms evaluated.