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

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Abstract
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 µg azithromycin disks, the following criteria are recommended: ⩾19 mm for the susceptible category (MIC⩽2.0 µg/ml) and ⩽15 mm for the resistant category (MIC⩾8.0 µg/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 strainStaphylococcus 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 µg/ml;Staphylococcus aureus ATCC 29213, 0.25–1.0 µg/ml; andEnterococcus faecalis ATCC 29212, 1.0–4.0 µg/ml.