The quality of antimicrobial discs from nine manufacturers-EUCAST evaluations in 2014 and 2017

Antimicrobial discs for susceptibility testing can be obtained from many manufacturers. We evaluated the quality of discs from nine manufacturers in 2014 and 2017.

Antimicrobial discs of 16 agents from nine manufacturers were evaluated using EUCAST criteria. Discs were tested in triplicate on Müller-Hinton medium against EUCAST quality control (QC) strains. Mean values were compared with targets and ranges in the EUCAST QC tables.

Three manufacturers (Becton Dickinson, Mast and Oxoid) demonstrated excellent and consistent disc quality both in 2014 and 2017. Manufacturers with discs of inadequate quality improved their results between the two periods. Overall, 92% (795/861) versus 97% (1038/1071) of zone diameter readings were within QC ranges and 58% (497/861) versus 75% (806/1071) were within the QC target ± 1 mm, for the first and second studies, respectively. One manufacturer (HiMedia) had major quality problems with 33% (26/78) of readings out of range in the first study and 17% (20/120) in the second study. Discs from some manufacturers showed unexpected variation in inhibition zone diameters (4-9 mm) for discs within the same vial.

Antimicrobial discs from three of nine manufacturers exhibited excellent and reproducible quality. The discs of the other six manufacturers demonstrated various quality issues, some of which were severe. After presenting the results to manufacturers and users, all managed to improve the quality. Our study points to the need for more stringent criteria for disc manufacturing. Criteria should not only address the nominal potency of discs but also define the end result.