The relationship of disc position to zone-of-inhibition size in the disc-agar diffusion test.

One hundred sixteen clinical isolates of Enterobacter sp., Escherichia coli, Klebsiella sp., Proteus mirabilis, Staphylococcus aureus and S. epidermidis were tested against cefoperazone, cefoxitin, cefamandole and piperacillin by disc-agar diffusion. Zones-of-inhibition of discs dropped near the center of the plate were compared with those dropped near the periphery. In 28% of the comparisons the inner and outer zones differed by 2 mm or more. Of those, the inner zone was larger 2.5 times more often than the outer zone. Results suggest that the chances of obtaining a susceptible result are greater when discs are placed near the center as opposed to the periphery of the plate.