Quality control parameters and interpretive criteria for in vitro susceptibility tests with the macrolide...

Quality control parameters for broth microdilution and disk diffusion susceptibility tests were defined and the interpretive criteria for disk diffusion tests reviewed. For interpretation of tests with 15 micrograms azithromycin disks, the following criteria are recommended: greater than or equal to 19 mm for the susceptible category (MIC less than or equal to 2.0 micrograms/ml) and less than or equal to 15 mm for the resistant category (MIC greater than or equal to 8.0 micrograms/ml). Using these criteria, there was 97% overall agreement between broth dilution and disk diffusion tests; Haemophilus influenzae isolates were susceptible to azithromycin by both methods. The quality control strain Staphylococcus aureus ATCC 25923 gave zones of 21 to 26 mm in diameter in a six-laboratory collaborative study. In azithromycin broth microdilution tests the following MIC control limits are recommended: Escherichia coli ATCC 25922, 2.0-8.0 micrograms/ml; Staphylococcus aureus ATCC 29213, 0.25-1.0 micrograms/ml; and Enterococcus faecalis ATCC 29212, 1.0-4.0 micrograms/ml.