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Intention to treat" is a strategy for the analysis of randomised controlled trials that compares patients in the groups to which they were originally randomly assigned. This is generally interpreted as including all patients, regardless of whether they actually satisfied the entry criteria, the treatment actually received, and subsequent withdrawal or deviation from the protocol. However there is a debate about the validity of excluding specific cases within each of these categories from an intention to treat analysis.1 Clinical effectiveness may be overestimated if an intention to treat analysis is not done.2
The intention to treat approach has two main purposes. Firstly, the approach maintains treatment groups that are similar apart from random variation. This is the reason for randomisation, and the feature may be lost if analysis is not performed on the groups produced by the randomisation process. For example, in a trial comparing medical and surgical treatment for stable angina pectoris, some patients allocated to surgical intervention died before being operated on.3 If these deaths are not attributed to surgical intervention using an intention to treat analysis, surgery seems to have a falsely low mortality (table 1). Secondly, intention to treat analysis allows for non-compliance and deviations from policy by clinicians. There are, of course, exceptions. Some types of deviations from randomised allocation may occur only within the trial setting and would not be expected in routine practice. For example, in a trial comparing active and placebo vaccination there is the potential for placebo vaccine to be incorrectly administered in place of active, but this could not occur outside the trial and so need not be accounted for in estimates of potential efficacy. However, most types of deviations from protocol would continue to occur in routine practice and so should be included in the estimated benefit of a change in treatment policy. Intention to treat analysis is therefore most suitable for pragmatic trials4 of effectiveness rather than for explanatory investigations of efficacy.
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Table 1. Use of intention to treat and other methods to analyse trial of coronary artery bypass surgery and medical treatment for stable angina pectoris in 768 men.2 Mortality 2 years after randomisation is shown by allocated and actual intervention*
Deviations from randomised allocation often result in missing outcome data. A full application of the intention to treat approach is possible only when complete outcome data are available for all randomised subjects. Care must always be taken to minimise missing responses and to follow up those who withdraw from treatment, but this is particularly important for the implementation of an intention to treat analysis.5 No consensus exists about how missing responses should be handled in intention to treat analyses, and different approaches may be appropriate in different situations. Practice also varies over handling of false inclusions (subjects found after randomisation not to satisfy the entry criteria). Thus, there is no single definition of an intention to treat analysis, and the phrase seems to have different meanings for different authors.6 We carried out a survey of recently published reports to examine current application of intention to treat analysis.
We identified all reports of randomised controlled trials published in 1997 in four major medical journals: BMJ, Lancet, JAMA, and New England Journal of Medicine. All except the New England Journal of Medicine have adopted the CONSORT statement,7 which requires that authors indicate whether analyses were performed on an intention to treat basis. The total number of randomised controlled trials was obtained by Medline searches for publication type "randomized controlled trials" within each journal and cross checked against the Cochrane controlled trials register.8 The journals were then hand searched to identify trials which reported an intention to treat analysis. For articles in the BMJ and Lancet, we also carried out a full text search for "intention to treat" or "intent to treat" on the internet (www.bmj.com, www.thelancet.com).
All trials that reported an intention to treat analysis were then independently assessed by both authors. We considered deviations from random allocation, false inclusions, and missing response. For each trial we recorded whether each of these occurred, and if so, the method of analysis and whether this method was explicitly stated. The assessment of missing response was limited to the primary outcomes if any were specified. Any uncertainties or disagreements between the two assessments were resolved by consensus.
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