Background
Antimicrobial susceptibility testing (AST) is a cornerstone of infection management. Cystic fibrosis (CF) treatment guidelines recommend AST to select antimicrobial treatments for CF airway infection but its utility in this setting has never been objectively demonstrated.

Methods
We conducted a systematic review of primary published articles designed to address two PICO (patient, intervention, comparator, outcome) questions: 1) “For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection predictable from AST results available at treatment initiation?” and 2) “For individuals with CF, is clinical response to antimicrobial treatment of bacterial airways infection affected by the method used to guide antimicrobial selection?” Relationships between AST results and clinical response (changes in pulmonary function, weight, signs and symptoms of respiratory tract infection, and time to next event) were assessed for each article and results were compared across articles when possible.

Results
Twenty-five articles describing the results of 20 separate studies, most of which described Pseudomonas aeruginosa treatment, were identified. Thirteen studies described pulmonary exacerbation (PEx) treatment and seven described ‘maintenance’ of chronic bacterial airways infection. In only three of 16 studies addressing PICO question #1 was there a suggestion that baseline bacterial isolate antimicrobial susceptibility was associated with clinical response to treatment. None of the four studies addressing PICO question #2 suggested that antimicrobial selection methods influenced clinical outcomes.

Conclusions
There is little evidence that AST predicts the clinical outcome of CF antimicrobial treatment, suggesting a need for careful consideration of current AST use by the CF community.