Quality control ranges for cefoperazone 30 μg disks for Staphylococcus aureus ATCC® 25923 and Escherichia coli ATCC® 25922

The third generation cephalosporin cefoperazone is commonly used for bovine mastitis therapy. So far, the cefoperazone susceptibility testing using 30 μg disks was hampered by the lack of quality control (QC) ranges and the respective interpretive criteria. The aim of this study was to develop CLSI-approved QC ranges for Staphylococcus aureus ATCC® 25923 and Escherichia coli ATCC® 25922. An interlaboratory trial including eight laboratories was conducted. Each laboratory tested both QC reference strains ten times using two lots of cefoperazone 30 μg disks and three lots of Mueller-Hinton agar. The results were analysed by using the published statistical analysis method. Based on the data, zone diameters of 23-34 mm and 24-33 mm were established as QC ranges for the 30 μg disk and S. aureus ATCC® 25923 and E. coli ATCC® 25922, respectively. These new QC ranges have been recently approved by the Clinical and Laboratory Standards Institute and will help diagnostic laboratories to validate their cefoperazone testing results using 30 μg disks.