Quality control of agar diffusion susceptibility tests. Data from the Quality Assurance Service Microbiology Program...

From 1974 through December 1978, over 180 laboratories participated in the Microbiology Program of the College of American Pathologists Quality Assurance Service (QAS), submitting a total of 2,372,000 individual antibiotic determinations on three quality control reference strains. Of these determinations, 89.5% were obtained by the standard Bauer-Kirby method; 8.4% by the agar overlay modification of Barry and associates. Standard statistical analysis of data obtained using the agar overlay modification has 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.