Comparison of the E test and a reference agar dilution method for susceptibility testing of anaerobic bacteria.

The susceptibility of 146 recent clinical isolates of gram-negative and gram-positive anaerobes was determined by the E test (AB Biodisk) on both Wilkins-Chalgren and PDM ASM II (AB Biodisk) agar. Results of the E test were compared with results obtained by the NCCLS agar dilution method using Wilkins-Chalgren agar. Incubation was for 20 hours and 44 hours in the E test and for 44 hours in the NCCLS method. In general, 44 hour results were more reliable; however, NCCLS readings were made only once after 44 hours. After two days of incubation, 91% of E test results on Wilkins-Chalgren agar were within one dilution and 98% within two dilutions of the corresponding NCCLS values; on PDM agar these values were 89% and 98%, respectively. Major and very major discrepancies combined were less than 1%.