Quality control limits for the agar overlay disk diffusion antimicrobial susceptibility test.

Replicate control tests were performed with the agar overlay and Kirby-Bauer techniques, using two sources of Mueller-Hinton agar. Mean zones observed with the two methods differed by 1 to 3 mm. Despite the minor differences between methods, National Committee for Clinical Laboratory Standards quality control limits which were established for the Kirby-Bauer method could also be used for the agar overlay technique.