BMJ 2005;331:942 (22 October), doi:10.1136/bmj.38624.397569.68 (published 13 October 2005)
Primary care
Bias from requiring explicit consent from all participants in observational research: prospective, population based study
Rustam Al-Shahi, specialist registrar in neurology1,
Céline Vousden, medical student1,
Charles Warlow, professor of medical neurology1, for the Scottish Intracranial Vascular Malformation Study (SIVMS) Steering Committee
1 Bramwell Dott Building, Division of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU
Correspondence to: Rustam Al-Shahi Rustam.Al-Shahi{at}ed.ac.uk
Objective To evaluate the differences between adults who consent to participate in observational research, and those who do not.
Design Prospective, population based cohort study.
Setting Primary and secondary care throughout Scotland.
Participants 187 adults (aged
16 years) resident in Scotland at the time of their first diagnosis of a brain arteriovenous malformation in 1999-2002.
Intervention Postal consent form sent via participants' general practitioner.
Main outcome measures Differences between consenters and non-consenters in demographic and clinical features at first presentation, and outcome during follow-up.
Results 111 adults (59%) consented to participate in the study. These consenters were not significantly different from non-consenters in age, sex, or socioeconomic status at first presentation. However, consenters were significantly more likely than non-consenters to present alive and independent, and with a seizure. During follow-up, consenters were significantly more likely to receive interventional treatment. Although consenters' survival was significantly better, they were more likely to have a seizure during follow-up. Presentation with intracranial haemorrhage conferred a higher risk of subsequent haemorrhage when the whole cohort was analysed, but not when it was restricted to consenters.
Conclusions We have found differences between adults who consent to participate in observational records-based research and those who do not, or cannot, consent. Blanket requirements for explicit consent for the use of individuals' identifiable data can bias disease registers, epidemiological studies, and health services research.

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