title: Bias from requiring explicit consent from all participants in observational research: prospective, population based study.
abstract: OBJECTIVE
  To evaluate the differences between adults who consent to participate in observational research, and those who do not.
  
  
  METHODS
  Prospective, population based cohort study.
  
  
  METHODS
  Primary and secondary care throughout Scotland.
  
  
  METHODS
  187 adults (aged > or = 16 years) resident in Scotland at the time of their first diagnosis of a brain arteriovenous malformation in 1999-2002.
  
  
  METHODS
  Postal consent form sent via participants' general practitioner.
  
  
  METHODS
  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.
authors.affiliation: Bramwell Dott Building, Division of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU. Rustam.Al-Shahi@ed.ac.uk
authors.forename: Rustam
authors.initials: R
authors.lastname: Al-Shahi
authors.forename: Céline
authors.initials: C
authors.lastname: Vousden
authors.forename: Charles
authors.initials: C
authors.lastname: Warlow
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D020785:Central Nervous System Vascular Malformations / Q000150:complications / Q000175:diagnosis* / Q000401:mortality; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D007258:Informed Consent; D020300:Intracranial Hemorrhages / Q000453:epidemiology / Q000209:etiology; D008297:Male; D008875:Middle Aged; D018579:Patient Selection / Q000941:ethics*; D011379:Prognosis; D012606:Scotland / Q000453:epidemiology; D012640:Seizures / Q000453:epidemiology / Q000209:etiology; D015983:Selection Bias