TABLE 1.
Polymyxin B and colistin interpretative breakpoints according to CLSI and EUCAST guidelines in 2018
| Microorganism | Polymyxin | CLSIa
,b
|
EUCASTa
|
||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Disk content | Zone diameter interpretative criteria (mm) |
MIC interpretative criteria (μg/ml) |
Disk content | Zone diameter interpretative criteria (mm) |
MIC interpretative criteria (μg/ml) |
||||||||||
| S | I | R | S | I | R | S | I | R | S | I | R | ||||
| Enterobacteriaceae | Polymyxin B | — | — | — | — | — | — | — | — | — | — | — | — | — | — |
| Colistin | — | — | — | — | — | — | — | — | — | — | — | ≤2 | — | >2 | |
| Pseudomonas spp. | Polymyxin B | — | — | — | — | ≤2 | 4 | ≥8 | — | — | — | — | — | — | — |
| Colistin | — | — | — | — | ≤2 | — | ≥4 | — | — | — | — | ≤2 | — | >2 | |
| Acinetobacter spp. | Polymyxin B | — | — | — | — | ≤2 | — | ≥4 | — | — | — | — | — | — | — |
| Colistin | — | — | — | — | ≤2 | — | ≥4 | — | — | — | — | ≤2 | — | >2 | |
S, susceptible; I, intermediate; R, resistant; —, not determined.
CLSI has recently established the MIC epidemiological cutoff value (ECV) for colistin applicable to five members of the Enterobacteriaceae, including E. coli, K. pneumoniae, K. aerogenes (formerly Enterobacter aerogenes), E. cloacae, and Raoultella ornithinolytica. ECVs are generally the MIC or zone diameter value that divides bacterial populations into those with and those without phenotypically detectable resistance (non-wild-type [NWT] and wild-type [WT], respectively). It is noteworthy that ECVs must not be used as clinical breakpoints for susceptible, intermediate, or resistant. If the colistin MIC for the above-mentioned members of the Enterobacteriaceae is at or below the ECV (≤2 μg/ml), it can be assumed that the isolate is a WT strain. If the colistin MIC is ≥4 μg/ml, the isolate should be retested, and if the result is confirmed, the isolate can be considered NWT.