Abstract: The search identified one multicentre study eligible for inclusion in the review. This study prospectively assessed whether the use of multiple combination bactericidal antibiotic testing improved clinical outcomes in participants with acute pulmonary exacerbations of cystic fibrosis who were infected with multiresistant bacteria. A total of 132 participants were randomised in the study. The study investigators provided data specific to the 82 participants who were only infected with Pseudomonas aeruginosa for their primary outcome of time until next pulmonary exacerbation. For participants specifically infected with only Pseudomonas aeruginosa, the hazard ratio of a subsequent exacerbation was 0.82, favouring the control group (95% confidence interval 0.44 to 1.51) (P = 0.52). No further data for any of this review's outcomes specific to participants infected with Pseudomonas aeruginosa were available. The risk of bias for the included study was deemed to be low. The quality of the evidence was moderate for the only outcome providing data solely for individuals with infection due to Pseudomonas aeruginosa. For other outcomes, we were unable to judge the quality of the evidence as no data were available for the relevant subset of participants. The current evidence, limited to one study, shows that there is insufficient evidence to determine effect of choosing antibiotics based on combination antimicrobial susceptibility testing compared to choosing antibiotics based on conventional antimicrobial susceptibility testing in the treatment of acute pulmonary exacerbations in people with cystic fibrosis with chronic Pseudomonas aeruginosa infection. A large international and multicentre study is needed to further investigate this issue. The only study included in the review was published in 2005, and we have not identified any further relevant studies up to March 2017. We therefore do not plan to update this review until new studies are published.

Plain-language summary: The search identified one study that tried to answer this question and was eligible for inclusion in the review. The study enrolled 132 people with cystic fibrosis, most of whom (82 people) had acute lung infections with Pseudomonas aeruginosa, and randomly put them into two treatment groups. In the first group two antibiotics were selected following the testing of combinations of antibiotics and in the second group the two antibiotics were chosen after testing individual antibiotics to see how effective the drugs were against the bacterium. The study was run across several centres and assessed the clinical outcomes in the participants after a 14-day course of treatment. The study investigators were only able to provide us with data for those who were infected with Pseudomonas aeruginosa for their main outcome (the time until the next acute lung infection). Choosing antibiotics based on the results of combination antibiotic testing did not lead to a longer time until the next lung infection compared to choosing antibiotics based on results of separate testing. They could not provide us with any results people infected with Pseudomonas aeruginosa for other outcomes in our review. We are satisfied that the people taking part were divided into the different treatment groups completely at random and no one could have foreseen which group any individual would be in. We are also satisfied that during the study, neither the individuals or clinic personnel knew which treatment group each individual was in. There were no missing data from the study. The quality of the evidence for the only outcome for which we have data (time to the next lung infection) is moderate, but we could not judge the quality of the evidence for other outcomes as there were no separate results available for people infected with Pseudomonas aeruginosa.