Quality assurance of antimicrobial susceptibility testing by disc diffusion

Quality assurance is essential to ensure the quality of antimicrobial susceptibility tests by diffusion methods. Routine internal quality control testing with a range of control strains is a major part of the quality assurance process, as it facilitates monitoring of the performance of the test. Most standardized methods include tables of acceptable zone size ranges for control strains and, in addition to checking that control zone diameters are within the published ranges, rules or statistical approaches may be applied to indicate deviations from acceptable performance. If control tests indicate unacceptable performance, the source(s) of the error should be investigated and may include problems with media, antimicrobial discs, inoculum and plate reading. Participation in external quality assessment schemes provides an independent assessment of performance although the number of strains distributed in such schemes is limited. Internal quality assessment in which routine tests are repeated with the identity of the organisms blinded is a useful complementary approach to external quality assessment and may detect problem areas not highlighted by other control methods. Education is an important part of the quality assurance process. Knowledge of atypical results for different organism-agent combinations may provide warning of possibly erroneous results, and an understanding of the limitations and sources of error in disc diffusion methods contributes significantly to the recognition, resolution and avoidance of errors.