A randomised trial of the effects on recruitment and retention of including a wet-ink signature and photograph in the patient invitation letter for a clinical trial: results from a Study Within a Trial (SWAT 3 and SWAT 53) [version 1; peer review: awaiting peer review]

Background: There is uncertainty about the effects of modifications to trial patient invitation letters on recruitment and retention. However, such documents are important.Methods: A 2x2 factorial design embedded a pair of randomised Studies Within a Trial (SWAT) within a large host trial (CLEAR), to evaluate the effects of including wet-ink signatures and photographs in the patient invitation letter. Patients were randomised to receive one of four invitation letters: with a wet-ink or generic signature, and with or without a photograph. The primary outcome was the proportion of invited patients who joined the CLEAR trial. The secondary outcome was the proportion of patients retained in the trial.Results: 368 letters were given to potential participants in the CLEAR trial and 121 (33%) joined. Proportions for each randomised group were generic signature and no photograph: 38% (33/88); generic signature and photograph: 32% (28/88); wet-ink personal signature and no photograph: 29% (26/91); wet-ink personal signature and photograph: 34% (34/101). There was no evidence of a significant difference in recruitment between those receiving the patient invitation letter containing a wet-ink versus generic signature (odds ratio (OR): 0.86, 95% confidence intervals (CI): 0.55 to 1.32, p=0.49) or photograph versus no photograph (OR: 0.99, 95% CI: 0.64 to 1.53, p=0.97). Retention was similar for the wet-ink and generic signature groups (OR: 1.20, 95% CI: 0.35 to 4.16, p=0.77) but significantly better when a photograph was used (OR: 5.40, 95% CI: 1.12 to 26.15, p=0.04, based on 2 withdrawals in the photograph group versus 9 in the no photograph group).Conclusions: These SWAT add to the evidence base for the effects of modifications to clinical trial documentation on recruitment and retention. We found including a photograph may improve retention. Although these analyses are underpowered, they will contribute to meta-analysis of similar comparisons.ISRCTN registration: 89040295 (06/07/2018)