Comparison of Etest, Vitek and Agar Dilution for Susceptibility Testing of Colistin

Clin Microbiol Infect. 2007 May;13(5):541-4. doi: 10.1111/j.1469-0691.2007.01708.x. Epub 2007 Mar 19.


In total, 172 isolates of Enterobacteriaceae, Acinetobacter spp., Pseudomonas aeruginosa and Stenotrophomonas maltophilia were tested for susceptibility to colistin by agar dilution, Etest and the Vitek 2 system. Isolates with a colistin MIC < or =2 mg/L were considered to be susceptible. Fifty-four (31%) Gram-negative isolates were resistant to colistin. Categorical agreement between agar dilution and Etest was 87%, and between agar dilution and Vitek 2 was 82%. Based on the data obtained, the Vitek 2 system was unreliable for detecting colistin resistance, and results obtained by Etest may require confirmation by a standard MIC susceptibility testing method.

Publication types

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

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Colistin / pharmacology*
  • Colony Count, Microbial
  • Enterobacteriaceae / drug effects
  • Gram-Negative Aerobic Rods and Cocci / drug effects
  • Humans
  • Microbial Sensitivity Tests / instrumentation*
  • Microbial Sensitivity Tests / methods*


  • Anti-Bacterial Agents
  • Colistin