With the increasing worldwide prevalence of multi-drug resistant (MDR) Gram negative bacteria, polymyxin has re-emerged as a last resort treatment. Polymyxin is a lipopeptide antibiotic isolated from Bacillus polymyxa. 

In 1975 the Calibrated Dichotomous Sensitivity (CDS) method recognised that polymyxin B had poor diffusibility in agar resulting in zones smaller than the standard 6mm annular radii. Thus, interpretation of polymyxin is based on an annular radius of 4mm or more for susceptibility, equivalent to a minimal inhibitory concentration of less than or equal to 1.0mg/ml.

Antibiotic disc susceptibility testing remains the most widely used method in clinical laboratories, but recently the accuracy of disc testing methods for polymyxins has come under scrutiny. While resistance to the agent may be intrinsic, chromosomally encoded or plasmid mediated, acquired resistance is still quite rare.

In recent times there has been numerous publications denouncing disc diffusion testing for the polymyxin family.  Many studies have focused on colistin (polymyxin E) and Mueller-Hinton agar.  Poriel et al observed that Iso-sensitest agar was more sensitive than Mueller-Hinton in detecting resistant subpopulations of Enterobacter cloacae isolates.  Sensitest agar used in CDS more closely resembles the composition of Iso-sensitest than Mueller-Hinton and in combination with the use of polymyxin B 300u discs may offer an advantage over other disc diffusion methods.

This review was undertaken to establish whether CDS users can continue to be confident in reporting polymyxin B according to their method. The study examined two aspects. First, an analysis of polymyxin B results from three major public hospitals in metropolitan and south eastern New South Wales. Data from 2007 - 2017 was assessed for prevalence of resistance. The second part of the evaluation involved a comparison of minimal inhibitory concentrations (MIC) determined by agar diffusion with disc diffusion and broth microdilution MICs.