I have found in the Bibliography that, due to poor and slow diffusion of colistin through agar, methods like disk diffusion (DD) or strips are not recomended to determine colistin MIC: Article Polymyxin susceptibility testing, interpretative breakpoints...
With this description I would dare to think that when you use these methods you will get false resistance as the antibiotic does not diffund properly and, in fact, I find many reviews that say things like "the poor and slow diffusion gives small zones of inhibition". However these reviews always conclude that in many studies where DD or strips have been compared with reference methods a high porcentaje of very major errors were found giving a lot of false sensitive strains. Therefore, how can be this phenomenon of poor diffusion supported by the results obtained?
I did some work with colistin and came across this piece of information. Since colistin property of poor diffusion is always there, consistent results is expected (reproducible results). However, diffusion is dependent on several variables; the purity of the agar, the thickness of the media and may be other factors. MIC procedures is the standard for detection of colstin.
As I understand it, the problem of disk diffusion is two-fold. One is the relatively large variation in diffusion and the other is small inhibition zone diameters. The zone diameter problem is that the difference between susceptible and resistant is very small and thus prone to interpretation errors. When you add the variability in diffusion, you're set up for trouble.
When it comes to strips (like Etest), I have no answer. The original developer at least used to say that the result is not dependent on diffusion, so that shouldn't explain the variability in results.
I think for agar dilution testing, if an isolate displayed a skipped-concentration phenomenon wherein no growth was observed at a given concentration of colistin (e.g. 0.25 mg/L) but growth was observed at higher concentrations (e.g. 0.5 and 1 mg/L), the MIC was read as one dilution above the highest concentration with growth (e.g. 2 mg/L).
Etest strips calibrated to a colistin concentration range of 0.064–1024 mg/L (first round of testing) or 0.016–256 mg/L (second and third rounds of testing) and Kirby Bauer (KB) antimicrobial discs containing 10 µg of colistin were used in accordance with recommendations of the manufacturers.
I think agar gel well dfiffusion method is more accurate than strips even the colistin is poorly diffused. Try to use different standarad agar and colistin brandname to select the best one for your test according to the manfacturers
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