Intended for healthcare professionals


Written informed consent and selection bias in observational studies using medical records: systematic review

BMJ 2009; 338 doi: (Published 12 March 2009) Cite this as: BMJ 2009;338:b866
  1. Michelle E Kho, registered physical therapist and PhD student1,
  2. Mark Duffett, clinical pharmacist and assistant professor2,
  3. Donald J Willison, associate professor1,
  4. Deborah J Cook, practising intensivist, clinical trialist, and professor13,
  5. Melissa C Brouwers, associate professor1, provincial director4
  1. 1Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4L8
  2. 2Department of Critical Care, McMaster Children’s Hospital, Hamilton, ON, Canada
  3. 3Department of Medicine, McMaster University, Hamilton, ON, Canada
  4. 4Program in Evidence-based Care, Cancer Care Ontario, McMaster University, Canada
  1. Correspondence to: M E Kho khome{at}
  • Accepted 2 December 2008


Objectives To determine whether informed consent introduces selection bias in prospective observational studies using data from medical records, and consent rates for such studies.

Design Systematic review.

Data sources Embase, Medline, and the Cochrane Library up to March 2008, reference lists from pertinent articles, and searches of electronic citations.

Study selection Prospective observational studies reporting characteristics of participants and non-participants approached for informed consent to use their medical records. Studies were selected independently in duplicate; a third reviewer resolved disagreements.

Data extraction Age, sex, race, education, income, or health status of participants and non-participants, the participation rate in each study, and susceptibility of these calculations to threats of selection and reporting bias.

Results Of 1650 citations 17 unique studies met inclusion criteria and had analysable data. Across all outcomes there were differences between participants and non-participants; however, there was a lack of consistency in the direction and the magnitude of effect. Of 161 604 eligible patients, 66.9% consented to use of data from their medical records.

Conclusions Significant differences between participants and non-participants may threaten the validity of results from observational studies that require consent for use of data from medical records. To ensure that legislation on privacy does not unduly bias observational studies using medical records, thoughtful decision making by research ethics boards on the need for mandatory consent is necessary.


  • Contributors: MEK conceived and designed the study and drafted the article. She had full access to all the data in the study, takes responsibility for the integrity of the data and the accuracy of the data analysis, and is guarantor. MEK and MD acquired the data; MEK, MD, and MCB analysed the data; and MEK, MD, DJC, DJW, and MCB interpreted the data. MD, DJC, DJW, and MCB critically revised the manuscript for important intellectual content. All authors approved the final version to be published.

  • MEK is funded by a fellowship from the Canadian Institutes of Health Research (Clinical Research Initiative). DJC is a research chair of the Canadian Institutes of Health Research. The Canadian Institutes of Health Research had no involvement in the design and conduct of the study; collection, management, analysis, and interpretation of the data; or preparation review, or approval of the manuscript.

  • Competing interests: None declared.

  • Ethical approval: Not required.

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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