The effects of a theory-informed cover letter on follow-up response rates in an adult cardiac surgery study: a randomised study within a trial (SWAT 24).

<ns3:p> Background Clinical studies can experience major loss to follow-up and reduced response rates to participant reported outcomes such as quality of life questionnaires, which can lead to a lack of statistical power for the analysis of these outcomes. A number of studies have been embedded within trials to investigate interventions to improve retention. Methods A Study Within a Trial (SWAT) was integrated into the Outcome Monitoring After Cardiac Surgery (OMACS) study, a large observational cardiac surgery study, to investigate whether the use of a theory-informed cover letter with 12-month follow-up questionnaires could improve study retention. OMACS study participants receiving 12-month follow-up questionnaires by post were randomised in a 1:1 ratio to receive either a standard cover letter or a theory informed cover letter with their questionnaires. Study retention was assessed by the proportion of participants who returned 12-month follow-up questionnaires. Results Questionnaire return rates were similar between the two groups (67.4% with the standard cover letter compared to 68.5% with the theory-informed cover letter, odds ratio 1.05, 95% confidence interval (CI) [0.85 to 1.31]). The results of the OMACS SWAT were consistent with the previously reported meta-analysis (MA) by Gillies <ns3:italic>et al.</ns3:italic> , with a revised risk difference of 3% (95% CI -1% to 6%) when including OMACS in the MA. Conclusions There is insufficient evidence to support the use of a theory-informed cover letter to increase study retention. We would recommend study teams explore alternative strategies for improving retention where needed. Trial registration: ISRCTN90204321, registered on 21/01/2015, https://doi.org/10.1186/ISRCTN90204321. </ns3:p>