Objective: To explore why reports that seem to describe randomised controlled trials are sometimes not indexed ("tagged") with RCT (randomised controlled trial) [pt] (publication type) in Medline. Design: Cross sectional study. Setting: The Cochrane Collaboration and US National Library of Medicine worked together to identify and retag records of randomised controlled trials with RCT [pt], 1994 to 2006. Data source: Published reports entered into Medline in 2005. Main outcome measures: Type of trial information presented (for example, main results, design, and methods), trial design, and other Medline indexing terms applied. Results: 572/591 (97%) untagged records and 578/594 (97%) tagged records contained information from randomised controlled trials. Type of trial information and design differed between untagged and tagged reports. Fewer than half (234/572, 41%, 95% confidence interval 37% to 45%) of untagged reports but most tagged reports (526/578, 91%, 89% to 93%) described the main results of the trial. Untagged reports were more likely than tagged reports to contain information on design and methods, baseline characteristics, long term follow-up, and secondary analyses. Untagged reports of main results were more likely than tagged reports to be from trials using a crossover design (36% v 10%, difference 25%, 95% confidence interval 19% to 32%). The Medical Subject Heading "Randomized Controlled Trials" was the most common clinical trial term applied to untagged reports, although more than half of untagged reports had no indexing related to trials. Conclusion: Based on the results for 2005, at least 3000 records describing randomised controlled trials but not indexed using RCT [pt] may have been entered into Medline between 2006 and 2011. Researchers and healthcare decision makers relying on using RCT [pt] may be missing important evidence in their searches, particularly for design and methods, baseline characteristics, long term follow-up, and secondary data analyses.
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