Comparison of the E-Test and Reference Agar Dilution Method for Susceptibility of Gram-Negative Anaerobic Organisms

Minimal inhibitory concentrations (MICs) of clindamycin, cefoxitin, imipenem, and metronidazole were determined by the E-Test and a reference agar dilution method for 92 gram-negative anaerobic organisms. For 335 MIC pairs, the agreement between the two systems was 80.1%; 60 (17.9%) differed by two or more twofold dilutions. The best agreement was observed with imipenem (91.7%) and the poorest with cefoxitin (67%). With the E-Test (PDM Epsilometer for antimicrobial susceptibility testing [AB Bodisk, Solna, Sweden]), MICs for individual organisms are simple to achieve, although for certain antibiotics discrepancies with the reference method are unacceptable.