Selective reporting of outcomes in randomised controlled trials in systematic reviews of cystic fibrosis.

Background: Outcome reporting bias (ORB) in randomised trials has been identified as a threat to the validity of systematic reviews. Previous work highlighting this problem is limited to considering a single primary review outcome. The aim of this study was to assess ORB across all efficacy outcomes in the Cochrane systematic reviews of cystic fibrosis.

Methods: Systematic reviews of interventions for cystic fibrosis published on the Cochrane Library by the Cochrane Cystic Fibrosis and Genetic Disorders Group before 2010 were assessed for discrepancies in outcomes between review protocol and full review. ORB in eligible trials was also assessed for all efficacy review outcomes. Two authors independently classified each outcome using a nine-point classification system developed by the Outcome Reporting Bias In Trials study. These classifications were used to inform the assessment of the risk of bias for selective outcome reporting for each trial.

Results: -46 Cochrane cystic fibrosis systematic reviews were included. The median number of primary outcomes, number of trials and participants per trial in the reviews were 3 (IQR 2, 3), 4 (IQR 2, 8) and 21 (IQR 14, 41), respectively. 18 reviews (39%, 18/46) had a discrepancy in outcomes between protocol and full review. 37 reviews were eligible to be included in the ORB assessment. When considering review primary outcomes and all review outcomes, ORB was suspected in at least one trial in 86% and 100%, respectively.

Conclusions: Assessment of ORB within a systematic review of a single primary outcome underestimates the risk of ORB in comparison to the assessment of multiple primary and secondary outcomes. ORB in trials is highly prevalent within systematic reviews of cystic fibrosis when assessed across all outcomes. This could be reduced by the development of a core outcome set for trials and systematic reviews in cystic fibrosis.