Effectiveness of theory-based invitations to improve attendance at cardiac rehabilitation: a randomized controlled trial. 
Despite well-established evidence of benefit from cardiac rehabilitation, typically fewer than 35% of eligible patients attend.  The purpose of this study was to evaluate whether theory-based invitations increase attendance at cardiac rehabilitation.  The study was a randomized controlled trial (RCT) with two by two factorial design.  A total of 375 participants with acute myocardial infarction or coronary revascularization was recruited from medical and surgical cardiac wards at Aberdeen Royal Infirmary (ARI).  They were randomly assigned to receive either the standard invitation letter or a letter with wording based on the 'theory of planned behavior (TPB)' and the 'common sense model of illness perception', and either a supportive leaflet with motivational messages or not.  The primary outcome was one or more attendances at cardiac rehabilitation.  The theory-based letter increased attendance at cardiac rehabilitation compared to the standard letter (84% versus 74%, odds ratio (OR) 2.93, 95% confidence interval (CI) 1.54-5.56), independent of age, gender, working status, hypertension, identity and TPB constructs.  The number needed to treat (NNT) was 9 (95% CI 7-12).  The motivational leaflet had no significant effect on attendance at rehabilitation (OR 1.02, 95% CI 0.57-1.83).  The use of theory-based wording in invitation letters is a simple method to improve attendance at cardiac rehabilitation.  Our letter, reproduced in this paper, could provide a template for practitioners and researchers.