Quality control of agar diffusion susceptibility tests: data from the Quality Assurance Service Microbiology program of the College of American Pathologists.

Over a 12-month period, between July 1981 and June 1982, 115 active participants in the Microbiology program of the College of American Pathologists Quality Assurance Service (QAS) submitted a total of 555,619 individual determinations on three quality control reference strains using the NCCLS standardized disc diffusion procedure. Data is presented for those antimicrobic agent/reference strain combinations for which NCCLS control limits have been changed since the last report of QAS microbiology data or that continue to show discrepancies with current NCCLS individual daily test control guidelines. Data for Escherichia coli versus cefoxitin, doxycycline, and nalidixic acid and for Staphylococcus aureus versus cefoxitin, nafcillin, and oxacillin show good compliance with the new NCCLS guidelines and distributions that are all approximately Gaussian. Significant discrepancies were noted for six combinations; cefamandole, cephalothin, neomycin, and nitrofurantoin versus E. coli and amikacin and clindamycin versus S. aureus. Of these discrepancies, only neomycin/E. coli and amikacin/S. aureus can be accounted for by a subpopulation of laboratories, which, when removed, corrects the data.