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

¹ Bramwell Dott Building, Division of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU

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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Rapid Responses:

Read all Rapid Responses

Yes, it could introduce bias.

Wenbin Liang
bmj.com, 17 Oct 2005 [Full text]

Statement of the bleedingly obvious

Peter D Singleton
bmj.com, 26 Oct 2005 [Full text]

Consent bias is important, but more research is needed

Kate M Dunn, et al.
bmj.com, 29 Oct 2005 [Full text]

to bias or not to bias

nicholas D. Moore
bmj.com, 31 Oct 2005 [Full text]