Regression analysis, proposed interpretative zone size standards, and quality control guidelines for a new macrolide...

A-56268 is a 6-O-methyl derivative of erythromycin A which has a spectrum of activity similar to that of erythromycin and is 1 log2 dilution more potent than erythromycin against most organisms that have been tested. The correlation of zone size diameters and MICs of A-56268 for 461 strains of bacteria isolated from clinical specimens was investigated. Based on anticipated levels in human serum of 2 micrograms/ml, 15-microgram disks have been recommended with zone size standards of greater than or equal to 15 mm for susceptibility (MIC correlate, less than or equal to 2.0 micrograms/ml) and less than or equal to 11 mm for resistance (MIC correlate, greater than or equal to 8 micrograms/ml). Selection of these tentative breakpoints resulted in no very major errors (false susceptible), a major error (false resistant) rate of 0.22%, and an acceptable minor error (intermediate) rate of 2.82%. MIC ranges and zone diameter limits for quality control organisms used in the standardized agar dilution and disk diffusion susceptibility tests with A-56268 are given.