What is meant by intention to treat analysis? Survey of published randomised controlled trials

BMJ 1999;319:670-674 ( 11 September )

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What is meant by intention to treat analysis? Survey of published randomised controlled trials

Sally Hollis, senior lecturer, ^a Fiona Campbell, statistician. ^b

^a Medical Statistics Unit, Fylde College, Lancaster University, Lancaster LA1 4YF, ^b AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG

Correspondence to: S Hollis s.hollis{at}lancs.ac.uk

Objectives: To assess the methodological quality of intentionto treat analysis as reported in randomised controlled trialsin four large medicaljournals.
Design: Survey of all reports of randomised controlledtrials published in 1997 in the BMJ, Lancet, JAMA, and New EnglandJournal of Medicine.
Main outcome measures: Methods of dealing with deviations from randomallocation and missingdata.
Results: 119 (48%) of the reports mentioned intentionto treat analysis. Of these, 12 excluded any patients who didnot start the allocated intervention and three did not analyseall randomised subjects as allocated. Five reports explicitlystated that there were no deviations from random allocation. Theremaining 99 reports seemed to analyse according to random allocation,but only 34 of these explicitly stated this. 89 (75%) trials hadsome missing data on the primary outcome variable. The methodsused to deal with this were generally inadequate, potentiallyleading to a biased treatment effect. 29 (24%) trials had morethan 10% of responses missing for the primary outcome, the methodsof handling the missing responses were similar in thissubset.
Conclusions: The intention to treat approach is often inadequatelydescribed and inadequately applied. Authors should explicitlydescribe the handling of deviations from randomised allocationand missing responses and discuss the potential effect of anymissing response. Readers should critically assess the validityof reported intention to treatanalyses.

Key messages

Intention to treat gives a pragmatic estimate of the benefit of a change in treatment policy rather than of potential benefit in patients who receive treatment exactly as planned
Full application of intention to treat is possible only when complete outcome data are available for all randomised subjects
About half of all published reports of randomised controlled trials stated that intention to treat was used, but handling of deviations from randomised allocation varied widely
Many trials had some missing data on the primary outcome variable, and methods used to deal with this were generally inadequate, potentially leading to bias
Intention to treat analyses are often inadequately described and inadequately applied

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