A third of systematic reviews changed or did not specify the primary outcome : A PROSPERO register study

OBJECTIVES: To examine outcome reporting bias of systematic reviews registered in PROSPERO. STUDY DESIGN AND SETTING: Retrospective cohort study. The primary outcomes from systematic review publications were compared with those reported in the corresponding PROSPERO records; discrepancies in the primary outcomes were assessed as upgrades, additions, omissions or downgrades. Relative risks (RR) and 95% confidence intervals (CI) were calculated to determine the likelihood of having a change in primary outcome when the meta-analysis result was favourable and statistically significant. RESULTS: 96 systematic reviews were published. A discrepancy in the primary outcome occurred in 32% of the included reviews and 39% of the reviews did not explicitly specify a primary outcome(s); 6% of the primary outcomes were omitted. There was no significant increased risk of adding/upgrading (RR 2.14, 95% CI 0.53 to 8.63) or decreased risk of downgrading (RR 0.76, 0.27-2.17) an outcome when the meta-analysis result was favourable and statistically significant. As well, there was no significant increased risk of adding/upgrading (RR 0.89, 0.31-2.53) or decreased risk of downgrading (RR 0.56, 0.29-1.08) an outcome when the conclusion was positive. CONCLUSIONS: We recommend review authors carefully consider primary outcome selection and journals are encouraged to focus acceptance on registered systematic reviews.