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

During the period from January 1979 through January 1981, 155 active participants in the Microbiology Program of the College of American Pathologists Quality Assurance Service (QAS) submitted approximately two million individuals determinations on three quality control reference strains. Of these determinations, 83% were obtained using the standardized disc-diffusion procedure of Bauer and co-workers, and, 6% using the agar overlay modification of Barry and associates. The mean and standard deviation for data obtained using these procedures were similar to those for data collected from the inception of the program in 1974 through December 1978. The number of individual determinations falling above or below existing control limits, however, was significantly less for the present data than for that previously reported. These differences result from a change in computation procedures. In the current report percentages of daily values exceeding the NCCLS limits were calculated using actual counts, whereas in the previous reports these percentages were estimated by accepting an assumption of Gaussian distribution.