Interpretive criteria and quality control for antimicrobial susceptibility tests of levofloxacin

To confirm preliminary interpretive breakpoints for prototype 5 μg levofloxacin disks, 490 strains were tested in vitro using commercially manufactured disks. For in vitro susceptibility testing, 5 μg levofloxacin disks can be used with interpretive criteria of ≤ 12 mm for resistant (MIC ≥ 8.0 μg/ml) and ≥ 16 mm for susceptible (MIC ≤ 2.0 μg/ml). Proposed quality control limits for tests of levofloxacin are as follows:Escherichia coli ATCC 25922, zones 29-37 mm or MIC 0.008-0.03 μg/ml;Pseudomonas aeruginosa ATCC 27853, zones 19-26 mm or MIC 0.5-2.0 μg/ml;Staphylococcus aureus ATCC 25923, zones 24-31 mm;Staphylococcus aureus ATCC 29213, MIC 0.06-0.25 μg/ml and Enterococcus faecalis ATCC 29212, MIC 0.25-2.0 μg/ml.