Majority of systematic reviews published in high-impact journals neglected to register the protocols: a meta-epidemiological study.

OBJECTIVES: To describe the registration of systematic review (SR) protocols and examine whether or not registration reduced the outcome reporting bias in high-impact journals.

STUDY DESIGN AND SETTING: We searched MEDLINE via PubMed to identify SRs of randomized controlled trials of interventions. We included SRs published between August 2009 and June 2015 in the 10 general and internal medicinal journals with the highest impact factors in 2013. We examined the proportion of SR protocol registration and investigated the relationship between registration and outcome reporting bias using multivariable logistic regression.

RESULTS: Among the 284 included reviews, 60 (21%) protocols were registered. The proportion of registration increased from 5.6% in 2009 to 27% in 2015 (P for trend <0.001). Protocol registration was not associated with outcome reporting bias (adjusted odds ratio [OR] 0.85, 95% confidence interval [CI] 0.39-1.86). The association between Preferred Reporting Items for Systematic review and Meta-Analysis (PRISMA) adherence and protocol registration was not statistically significant (OR 1.09, 95% CI 0.59-2.01).

CONCLUSIONS: Six years after the launch of the PRISMA statement, the proportion of protocol registration in high-impact journals has increased some but remains low. The present study found no evidence suggesting that protocol registration reduced outcome reporting bias.