The acquired mutational resistance mechanism in MAC isolates (ie. 23SrRNA gene mutation) is the same for both clarithromycin and azithromycin. Therefore, to be cost effective, only one macrolide needs to be tested. Because of technical difficulties associated with testing azithromycin (ie, poor solubility at the high drug concentrations that need to be tested), clarithromycin is the most appropriate class drug for testing the macrolides. Azithromycin AST is not recommended.