Identification of Additional Trials in Prospective Trial Registers for Cochrane Systematic Reviews

Abstract

BackgroundPublication and selective outcome reporting bias are a threat to the validity of systematic reviews. Extensive searching for additional trials in prospective trial registers could reduce this problem. We have evaluated how authors of Cochrane systematic reviews currently make use of trial registers as an additional source for the identification of potentially eligible trials.Methodology/Principal FindingsWe included 210 systematic Cochrane reviews of interventions published between 2008 and 2010 of which the protocol was first published in 2008. When prospective trial registers were searched we recorded the names of the register(s), the authors' motive(s) and if they yielded any extra trials.In 80 reviews (38.1%) the authors had searched in one or more prospective trial register(s) of which 55% had searched in overlapping search portals and individual registers. Most frequently assessed were the MetaRegister (66.3%) and Clinicaltrials.gov (60%) which is in sharp contrast of other registers or portals like the WHO ICTRP Search Portal (20%). Reported motives to use registers were to identify ongoing trials (83.3%), to identify unpublished outcomes or trials (23.5%), to identify recently published trials (11.8%), or to identify any relevant trial (3.9%).In 28 reviews (35%) the authors had selected (ongoing) trials identified in trial registers as potentially eligible.DiscussionTrial registers as an additional source of information are gaining acknowledgement amongst Cochrane reviewers. Nevertheless, searches seem to be inefficient as overlapping databases are frequently consulted, while the WHO ICTRP Search Portal that includes the data from all approved registers worldwide is being underused. Moreover, the emphasis is now on the identification of ongoing trials, although the prospective registers offer a broader potential. Further familiarity of registers and guidance how to search and to report will help to implement this as a common method and utilize the full potential of prospective trial registers for systematic reviews.

Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015: elaboration and explanation

“…81 If it is planned to filter out (via search filter, seeItem 10) or exclude specific types of records (such as commentaries, letters, editorials, etc) during screening, this should be stated. sources Item 9.…”

Trends in global clinical trial registration: an analysis of numbers of registered clinical trials in different parts of the world from 2004 to 2013

“…A key example of this is that trial registration is already helping to address the pertinent problems of publication bias and selective reporting bias by allowing reviewers to find non-published trials and non-published outcomes when conducting systematic reviews. 20 21 …”

A comparison of results of empirical studies of supplementary search techniques and recommendations in review methodology handbooks: a methodological review

“…The handbooks focus on the benefit of searching registers [ 10 ], with The Cochrane Handbook providing specific guidance on where to search [ 9 ]. The studies focused on the searching of the registers [ 38 ] and the advantages and disadvantages of doing so [ 39 , 40 ]. Three studies were included [ 38 – 40 ].…”

“…It is used for identifying unpublished, recently completed or on-going trials [ 9 , 10 , 39 , 40 ] and keeping a track of any adaptations to trial protocols and reported study outcomes [ 39 , 40 ]. Trials that have been stopped, or were unable to reach optimal recruitment, can also be identified.…”