Standardisation of disk diffusion results for antibiotic susceptibility testing using the sirscan automated zone reader

BACKGROUND: Standardisation of disk diffusion readings could improve reproducibility and accuracy of antibiotic susceptibility testing (AST). This study evaluated accuracy, reproducibility, and precision of automated inhibition zone reading using the “Sirscan automatic” zone reader (i2a, Perols Cedex, France). RESULTS: In a first step we compared Sirscan results with manual calliper measurements for comparability and accuracy. Sirscan readings were checked and adjusted on-screen as recommended by the manufacturer. One hundred clinical bacterial isolates representing a broad spectrum of organisms routinely isolated in a clinical laboratory were tested, and zone diameter values and interpretation according to EUCAST guidelines were compared. In a second step we analysed, whether fully automated zone reading can decrease standard deviation of diameter measurements and, thus, improve reproducibility and precision of the disk diffusion method. Standard deviations of manual measurements, on-screen adjusted Sirscan measurements, and fully automated Sirscan readings were compared for 19 repeat independent measurements of inhibition zones of S. aureus ATCC 29213, E. coli ATCC 25922, and P. aeruginosa ATCC 27853 (EUCAST quality control strains). On-screen adjusted Sirscan and calliper measurements displayed high comparability. No significant differences were detected comparing the results of both reading methods. Standard deviations of inhibition zone diameters were significantly lower for fully automated Sirscan measurements compared with both adjusted Sirscan readings and the manual method, resulting in better reproducibility and precision of the automated readings. CONCLUSIONS: Our results indicate that fully automated zone reading can further improve standardisation of AST by decreasing standard deviation and, thus, improve precision of inhibition zone diameter results.