Interpretative criteria for the agar diffusion susceptibility test with azithromycin.

Azithromycin (CP-62,993) is a macrolide with a tendency to concentrate in various body tissues. Difficulties that arise when establishing tentative minimum inhibitory concentration (MIC) breakpoints for such a drug include the question of whether MICs should be compared with the achievable concentrations in blood or in tissue. Pending clinical experience with this drug, we recommend the following tentative criteria for interpreting tests with 15 micrograms azithromycin discs: susceptible greater than or equal to 18 mm (MIC less than or equal to 2 mg/l) and resistant less than or equal to 13 mm (MIC greater than or equal to 8 mg/l).