Background: Conflicts of interest have the potential to bias the findings of systematic reviews.
Objectives: The objective of this methodological survey was to assess the frequency and types of conflicts of interest that authors of Cochrane and non-Cochrane systematic reviews report.
Methods: We used standard systematic review methodology. We searched for systematic reviews using the Cochrane Database of Systematic Reviews and Ovid MEDLINE (limited to the 119 Core Clinical Journals and the year 2015). We defined a conflict of interest (COI) disclosure as the reporting of whether a COI exists or not, and used a framework to classify COIs into individual (financial, professional, and intellectual) and institutional (financial and advocatory) COIs. We conducted descriptive and regression analyses.
Results: Of the 200 systematic reviews we included, 194 (97%) reported authors’ COI disclosures, typically in the main document, and in a few cases either online (2%) or upon request (5%). Of the 194 Cochrane and non-Cochrane reviews, 49% and 33% respectively had at least one author reporting any type of COI (P = 0.023). Institutional COIs were less frequently reported than individual COIs, and Cochrane Reviews were more likely to report individual intellectual COIs compared to non-Cochrane reviews (19% and 5% respectively, P = 0.004). Regression analyses showed a positive association between reporting of COIs (at least one type of COI, individual financial COIs, institutional financial COIs) and journal impact factor, and between reporting individual financial COIs and pharmacological versus non-pharmacological intervention.
Conclusions: Conflicts of interest, reported in close to half of published systematic reviews (typically many authors) constitute a potentially problematic source of bias in the conduct, reporting, and conclusions of systematic reviews. The association with journal impact factor suggests the possibility that authors publishing their reviews in lower impact journals fail to report existing conflicts of interest. This raises the need for a wider and standardized reporting of conflict of interest disclosures.