The Sense and Nonsense of Antimicrobial Susceptibility Testing in Cystic Fibrosis.

Antimicrobial susceptibility testing (AST) has been used to guide therapy of airway infection in persons with cystic fibrosis (CF) for decades. However, evidence that AST adds benefit to treatment outcomes in CF is lacking. In fact, the routine use of AST has potential to exacerbate inappropriate antibiotic use. Several features of airway infection in CF contribute to the limitations of AST in predicting treatment outcomes, providing rationale for abandoning this practice altogether. Other features of CF infection suggest, however, that select use of AST can provide worthwhile guidance to antibiotic selection.