Comparison of 3 Etest(®) methods and time-kill assay for determination of antimicrobial synergy against carbapenemase-producing Klebsiella species

Diagn Microbiol Infect Dis. 2013 Nov;77(3):220-6. doi: 10.1016/j.diagmicrobio.2013.07.006. Epub 2013 Aug 28.


Increasing global antibiotic resistance has resulted in more use of antibiotic combinations. There is a lack of a gold standard for in vitro testing of these combinations for synergy or antagonism. Time-kill assay (TKA) may be used but is labor intensive and not practical for clinical use. Etest® synergy methods are more rapid and easier to perform, but there is no agreement regarding which method is best. We tested 31 clinical genetically unique Klebsiella pneumoniae carbapenemase-producing Klebsiella isolates with the combination of meropenem and polymyxin B by TKA and 3 Etest methods, each in triplicate: Method 1, MIC:MIC; Method 2, direct overlay; and Method 3, cross. Overall, testing with Etest synergy methods showed the following agreement with TKA: Method 1: 25/31 (80.6%), Method 2: 7/31 (22.6%), and Method 3: 8/31 (25.8%). The MIC:MIC method had the highest agreement (80.6%, κ = 0.59, P < 0.001) and should be evaluated more extensively.

Keywords: Etest; Klebsiella; Synergy.

Publication types

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

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Bacterial Proteins / metabolism*
  • Drug Synergism*
  • Humans
  • Klebsiella Infections / microbiology
  • Klebsiella pneumoniae / drug effects*
  • Klebsiella pneumoniae / enzymology*
  • Klebsiella pneumoniae / isolation & purification
  • Microbial Sensitivity Tests / methods*
  • beta-Lactamases / metabolism*


  • Anti-Bacterial Agents
  • Bacterial Proteins
  • beta-Lactamases
  • carbapenemase