Microbiological assay for azithromycin in pharmaceutical formulations.

The validation of a microbiological assay, applying the cylinder-plate method, for the determination of the antibiotic azithromycin is described. Using a strain of Micrococcus luteus ATCC 9341 as the test organism, azithromycin at concentrations ranging from 0.1 to 0.4 microgml(-1) could be measured in capsules and suspensions. A prospective validation of the method showed that it was linear (r=0.998), precise (RSD=1.40-capsules; RSD=1.19-powder for suspension and RSD=1.73-oral suspension) and accurate (it measured the added quantities). We conclude that the microbiological assay is satisfactory for quantitation of in vitro antibacterial activity of azithromycin.