Background: Insufficient blinding of persons involved in randomized clinical trials is associated with bias. The appraisal of the risk of bias is difficult without adequate information in trial reports.
Purpose: We wanted to study how blinding is reported in clinical trials and how lack of reporting relate to lack of blinding.
Methods: A cohort study of 200 blinded randomized clinical trials published in 2001 randomly sampled from the Cochrane Central Register of Controlled Trials, and a questionnaire survey of the trial authors.
Results: One-hundred and fifty-six (78%) articles described trials as 'double blind'. In three (2%) of such articles the blinding status of patients, health care providers and data collectors was explicitly described. Eighty-eight (56%) articles did not describe the blinding status of any trial person, and 41 articles (26%) reported no blinding relevant information at all beyond the trial being 'double blind'. One-hundred and thirty (65%) surveyed authors responded. Patients were blinded in 101 (97%) 'double blind' trials, and health care providers in 93 (89%). Twenty (19%) 'double blind' trials had not blinded either patients, health care providers or data collectors. Survey responders provided 15 different operational meanings of the term 'double blind', and typically felt that their preferred definition was the most widely used.
Limitations: The proportions in the author survey may be too optimistic due to reporting bias. It is not known how the increased use of the CONSORT guidelines may have affected reporting in years after 2001.
Conclusions: The blinding status of key trial persons was incompletely reported in most randomized clinical trials. Unreported blinding may be frequent, but one of five 'double blind' trials did not blind either patients, treatment providers or data collectors. Authors, referees, and journal editors could improve the completeness of reporting of blinding, eg, by adhering to the CONSORT statement. It is inappropriate to presume blinding of key trial persons based only on the ambiguous term 'double blind'.