Evaluation of E Test, disk diffusion and broth microdilution to establish tentative quality control limits and review susceptibility breakpoints for two aerobic actinomycetes

A single laboratory study was carried out to compare E Test with broth micr odilution and disk diffusion to establish tentative quality control ranges for Nocardia asteroides ATCC 19247 and Rhodococcus equi ATCC 6939 against a panel of eight antimicrobial agents. Reproducibility testing was performed on 12 consecutive days to establish tentative quality control ranges. A to tal of 36 clinical strains of the Nocardia asteroides complex and 5 Rhodoco ccus strains were used in the study. Both candidate control strains and cli nical strains grew well on cation-adjusted Mueller-Hinton agar. Adequate gr owth occurred at 48 to 72 h for the Nocardia isolates and 24 to 48 h for Rh odococcus. A standardized primary inoculum of 5 X 10(4) CFU/mL was used for performance of E Test and disk diffusion for the Nocardia isolates. Tentat ive population-based error rates were calculated using current breakpoints for Enterobacteriaceae for E Test compared with disk diffusion for the 36 c linical strains of Nocardia species. Significant very major error rates wer e observed for imipenem (22%) and minor error rates varied from 2.7% to 50% . These methods require more extensive validation before definitive breakpo int criteria can be established. (C) 2001 Elsevier Science Inc. All rights reserved.