Intended for healthcare professionals

Education And Debate

Modified informed consent procedure: consent to postponed information

BMJ 2003; 327 doi: (Published 31 July 2003) Cite this as: BMJ 2003;327:284
  1. Han Boter, junior researcher (,
  2. Johannes J M van Delden, professor of medical ethics2,
  3. Rob J de Haan, professor of clinical epidemiology3,
  4. Gabriël J E Rinkel, professor of neurology1
  1. 1Department of Neurology, Rudolf Magnus Institute of Neuroscience, University Medical Centre Utrecht, PO Box 85500, 3508 GA Utrecht, Netherlands
  2. 2Julius Centre for Health Sciences and Primary Care, University Medical Centre Utrecht
  3. 3Department of Clinical Epidemiology and Biostatistics, Academic Medical Centre, PO Box 22660, 1100 DD Amsterdam, Netherlands
  1. Correspondence to: H Boter
  • Accepted 19 May 2003

How do you obtain a valid assessment of subjective outcomes in a trial in which the participants cannot be blinded to the intervention? Bias is inevitable from unblinded patients, but trials that have not told patients about treatment in all arms have been heavily criticised. Asking participants to consent to postponed information could be a solution

The most powerful tool for studying the effectiveness of a medical treatment is a randomised controlled clinical trial with blinded assessment of outcomes. However, blinding is not always possible—for example, in a trial comparing a surgical intervention with non-surgical treatment or the effectiveness of supplemental care compared with conventional care. Blinding needs special attention in such studies in order to prevent bias. When outcomes are assessed by doctors, it is often easy for the assessment to be done by doctors other than those who performed the procedure. However, when studies measure patients' assessment of outcomes, blinding is much more complicated or even impossible.

Embedded Image


Dealing with unblinded participants

Unblinded patients who assess outcomes after being informed about the different treatment options during recruitment might bias the results of a study. The likelihood of bias increases when patients have a preference for one of the treatment options. For example, patients may have been told during recruitment that the new or supplemental strategy has been developed because the current strategy has disadvantages. The best way to obtain valid assessments in such studies remains unclear.

Intense debate was generated when researchers tried to mask patients …

View Full Text

Log in

Log in through your institution


* For online subscription