Accuracy of the E test for determining antimicrobial susceptibilities of staphylococci, enterococci, Campylobacter jejuni, and gram-negative bacteria resistant to antimicrobial agents.

We compared the results of the E test MIC method with the results of agar dilution susceptibility testing for 18 antimicrobial agents against 324 strains of gram-positive and gram-negative bacteria, including 99 strains of staphylococci, 101 strains of antimicrobial-resistant gram-negative bacteria, 40 strains of enterococci, and 84 isolates of Campylobacter jejuni. Overall agreement of MICs (+/- 1 log2 dilution) was 97.3% for staphylococci, 94.6% for gram-negative bacilli, and 100.0% for enterococci. The MIC results for C. jejuni showed an overall agreement of only 82.9%. This was due primarily to a number of offscale values that limited the number of comparisons with clindamycin, trimethoprim-sulfamethoxazole, and tetracycline. Interpretative criteria for the results of the two test methods, however, were similar. Overall, the E test produced MIC results comparable to those of agar dilution when multiresistant organisms were tested. However, it was necessary to add 2% NaCl to the agar when testing oxacillin against staphylococci for both the E test and agar dilution to obtain results comparable to those of the broth microdilution method.