Challenges of identifying unpublished data from clinical trials: Getting the best out of clinical trials registers...

Clinical trial data are essential for assessments of the effectiveness of health care interventions. Information about ongoing or completed, but not yet formally published, trials has been more difficult to identify until the development of clinical trials registers and portals. This paper summarises research evidence on identifying sources of trial data, how and when to search those sources, and which future developments may enhance access to and retrieval of unpublished trial evidence. We conducted a literature search for relevant studies and provide a narrative review of the evidence from these studies. Clinical trial data can be found in resources including clinical trials registers, regulatory agency sources, health technology assessment websites and manufacturers' websites, and submissions for regulatory approval. The challenges of searching these resources are described. Trials registers are relatively unsophisticated in terms of their search interfaces, and searchers need to adapt to each individual register. There is overlap across registers, but little research on the degree and nature of overlap and how best to search. Despite these challenges, trials registers and other resources can be rich sources of additional unique trial data, which may not be available from journal reports. New initiatives, such as OpenTrials, aim to consolidate and link all structured data and documentation related to clinical trials. No single resource gives access to all trials, and multiple registers should be searched as sensitively as possible. Searching is challenging and should be adequately resourced. Information specialists should monitor new developments which may reduce the challenges over the coming years.