TABLE 1

Colistin and polymyxin B breakpoints according to CLSI and EUCAST in 2014

Criteria and bacterial groupColistinPolymyxin B
Disk content (μg)Zone diam interpretative criteria (mm)aMIC interpretative criteria (μg/ml)aDisk content (IU)Zone diam interpretative criteria (mm)aMIC interpretative criteria (μg/ml)a
Sensitive (S)Resistant (R)SIntermediate (I)RSRSIR
CLSI criteria
    Enterobacteriaceae300bb
    Acinetobacter spp.≤2≥4bb≤2≥4
    Pseudomonas spp.10≥11≤10≤24≥8300≥12≤11≤24≥8
EUCAST criteria
    Enterobacteriaceae50≥18b,c<15b,c≤2>2bb≤2>2
    Acinetobacter spp.cc≤2>2bb≤2>2
    Pseudomonas spp.cc≤4>4bb
  • a —, not determined or absent.

  • b Zone diameter interpretative criteria for Enterobacteriaceae given by EUCAST are only for veterinary medicine; MIC determination is required for diameters ranging from 15 to 18 mm.

  • c No zone diameter interpretative criteria for human medicine; the MIC must be determined before use.