Published 12 March 2009, doi:10.1136/bmj.b866
Cite this as: BMJ 2009;338:b866

Research

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

¹ Department of Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4L8, ² Department of Critical Care, McMaster Children’s Hospital, Hamilton, ON, Canada, ³ Department of Medicine, McMaster University, Hamilton, ON, Canada, ⁴ Program in Evidence-based Care, Cancer Care Ontario, McMaster University, Canada

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.

© Kho et al 2009
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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