Quality control of agar diffusion susceptibility tests

Since its beginning in 1974 over 180 laboratories have participated in the Microbiology Program of The College of American Pathologists Quality Assurance Service submitting a total of 2,372,000 individual antibiotic determinations on three quality control reference stains. Eighty-nine and five tenths percent of these determinations were obtained using the standard Bauer-Kirby method; 8.4% using the agar overlay modification of Barry and associates. Standard statistical analysis of data obtained using the Bauer-Kirby method have been reported for each antimicrobic/reference strain combination. Comparisons have been made between the QAS data and those data obtained in earlier collaborative studies which currently serve as precision and accuracy control limits. In many cases QAS data exceed the existing control limits.