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

Diagn Microbiol Infect Dis. 2001 Aug;40(4):179-86. doi: 10.1016/s0732-8893(01)00273-5.


A single laboratory study was carried out to compare E Test with broth microdilution 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 total of 36 clinical strains of the Nocardia asteroides complex and 5 Rhodococcus strains were used in the study. Both candidate control strains and clinical strains grew well on cation-adjusted Mueller-Hinton agar. Adequate growth occurred at 48 to 72 h for the Nocardia isolates and 24 to 48 h for Rhodococcus. 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. Tentative population-based error rates were calculated using current breakpoints for Enterobacteriaceae for E Test compared with disk diffusion for the 36 clinical strains of Nocardia species. Significant very major error rates were observed for imipenem (22%) and minor error rates varied from 2.7% to 50%. These methods require more extensive validation before definitive breakpoint criteria can be established.

Publication types

  • Comparative Study
  • Evaluation Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aerobiosis
  • Culture Media
  • Diffusion
  • Microbial Sensitivity Tests / methods*
  • Microbial Sensitivity Tests / standards
  • Nocardia / drug effects*
  • Quality Control
  • Reproducibility of Results
  • Rhodococcus / drug effects*


  • Culture Media