Alternative quality control parameters for Autobac susceptibility testing disks: use of agar diffusion zone size...

Persistent concerns about limitation in the assessment of the quality control of Autobac 1 (Pfizer Diagnostics Division, Groton, Conn.) led us to investigate an alternative method for monitoring the performance of Autobac susceptibility testing disks. Current methodology for quality control of the system provides data which are interpreted at the high end of a numerical scale; e.g., the control strain of Escherichia coli consistently exhibits a light-scattering index value of 1.00 for all antibiotics tested. This type of end-of-scale criterion may not detect individual antibiotic disk aberrations of individual clinical isolate susceptibilities. Disk diffusion testing allows a semiquantitative, continuous-scale determination and will detect test performance variations, unless the control strain is highly resistant. During a 6-month period daily quality control procedures for 10 Autobac antibiotics tested against control strains of E. coli and Pseudomonas aeruginosa were monitored, utilizing both Autobac 1 recommendations and disk diffusion susceptibility (Kirby-Bauer) methodology. Readings were carried out by one of six technologists. Zone sizes were within a range of +/- 3 mm of a mean value of 99% of the tests with E. coli and within +/- 3 mm for 98% of the tests with Pseudomonas. Reproducibility was excellent. The high reproducibility may be due to the disk manufacturing process, which provides rigorous disk preparation and acceptability standards, to strict laboratory storage procedure, and to our own careful assessment of disk cartridges before their use for clinical susceptibility testing. We recommend that each new cartridge be tested in this manner and that a similar procedure be considered for other automated procedures in which disks are used.