Breakpoints of colistin

The EUCAST breakpoints are based on new PK and clinical information and apply to a colistin dosage of 2-3 Mill IU x 3 with a loading dose (9 Mill IU). Using the EUCAST breakpoints require an adequate dosing that is not usual practice in all countries. Considering the growing knowledge but also unsolved issues the EUCAST committee is assessing the current breakpoints of colistin.

Determinig reliable MIC values of colistin is fraught with complications, due in part to the inherent cationic properties of colistin that promote the tendency to precipitate and bind avidly to plastic materials. The MIC results are greatly influenced by the characteristics of the polystyrene microtitre trays (M. Albur et al, 2013). In addition, no reference method has been defined against which to compare the results of colistin susceptibility testing. The addition of polysorbate 80, a surfactant, is not universally agreed on but favored by some microbiologists (HS Sader et al 2012) and rejected by others (M Albur et al 2013). MIC results are greatly influenced by the characteristics of different types of polystyrene microtitre trays (M Albur et al 2013). Thus, variable results can occur when using different methods for colistin MIC testing. Especially, MICs determined by E-test have low agreement with other routine methods (J.A. Hindler et al 2013). Susceptibility also should not be assumed based on the results of a single test as conflicting or uninterpretable results are common (D. Landman et al 2013)

Several studies have found disk diffusion to be an unreliable method to measure susceptibility to colistin. High error rates, as well as a low level of reproducibility between subsequent measurements for the same isolate have been reported (JR Lo-Ten-Foe et al 2007).

A review of the methodology for colistin MIC testing is urgently needed.

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