A nine-laboratory cooperative study was performed to evaluate quality control for susceptibility tests with 30-?g netilmicin disks. Each laboratory tested three standard control strains on a different lot of Mueller-Hinton agar, with 150 tests performed per laboratory. For protocol control, each laboratory also performed 15 additional tests on each of the control strains using thesamelot of Mueller-Hinton agar across laboratories. The mean zone diameters for each of the nine laboratories are given below.

Mean zone diameters with 30-ug netilmicin disks tested in nine separate laboratories Type of control strain E. coli S. aureus P. aeruginosa Laboratory Different media Common medium Different media Common medium Different media Common medium 27.5 23.8 25.4 23.9 20.1 16.7 24.6 21.1 24.8 24.2 18.4 17.0 25.3 25.4 24.6 25.0 16.8 17.1 28.7 25.4 29.8 26.7 21.7 18.2 -IOTMOODD 23.0 24.8 27.5 25.3 20.1 16.7 26.8 25.7 28.1 25.2 20.3 19.2 24.7 26.8 31.2 27.1 22.8 18.8 24.3 26.2 24.3 26.5 19.9 18.1 24.9 26.3 25.4 25.1 19.3 19.2