Dirithromycin disc susceptibility tests: interpretative criteria and quality control parameters

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Abstract
Dirithromycin and its bioactive metabolite (erythromycylamine) were compared in vitro to erythromycin against 450 bacterial isolates. Dirithromycin and erythro-mycylamine did not differ in their in-vitro activity and the two compounds were additive when combined in equal proportions. Both compounds were active against erythromycin-susceptible bacteria but erythromycin was two to four times more potent on a weight-to-weight basis. Interpretive criteria for dirithromycin were not based on achievable blood concentrations since tissue concentrations may be 20–30 times greater than peak serum levels. Dirithromycin-susceptible strains were those with zones ≥ 19 mm in diameter (15 μg dirithromycin disc) or MIC ≤2.0mg/L and resistance was defined as a zone ≤ 15 mm or MIC ≥ 8.0 mg/L. Interpretive criteria for testing Haemophilus influenzae on Haemophilias Test Medium (HTM) agar were not defined because of the drug's poor activity in the test system used. Two multi-laboratory studies were also undertaken, one to define quality control limits for disc diffusion tests and the other to define broth microdilution MIC control limits for tests with dirithromycin.

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