Abstract
Background: Retention is considered the second highest trial methods priority in the UK after recruitment. Methods: This Study Within A Trial (SWAT) evaluated whether sending a pre-notification card around one month before a face-to-face primary outcome measurement visit compared to not sending the card increased trial retention. The SWAT was a two-arm, parallel randomised (1:1 allocation ratio), stratified by centre, study. It was embedded within the ActWELL host trial, which evaluated whether women receiving lifestyle change counselling from volunteer coaches improved outcomes including weight and physical activity. The SWAT primary outcome was the difference in the proportion of participants attending the host trial primary outcome measurement visit. The secondary outcome was the direct cost of sending cards. Host trial participants and research staff at the primary outcome visit were blind to the SWAT. Analysis was intention-to-treat. Certainty of evidence was assessed using GRADE. Results: 558 host trial participants took part in the SWAT and were included in the analysis. Of the 274 women sent a card, 231 attended the primary outcome visit (84.3%) compared to 230/284 (81.0%) for those not receiving a card. Sending a pre-notification card may result in a slight increase in attendance at a face-to-face primary outcome measurement visit at 1-year: risk difference = 3.3% (95% confidence interval = -3.0% to 9.6%). This is GRADE low certainty evidence. A recording error meant it was unclear whether 17 participants allocated to the card were actually sent one but a sensitivity analysis did not change the overall result or conclusion. The direct cost of producing and sending the cards was £192 GBP (€213 EUR; $260 USD), or £21.33 (€23.55; $28.77 per additional retained participant. Discussion: Trialists could consider using pre-notification as they may gain a slight increase in retention to face-to-face trial measurement visits but further evaluations are needed.
Highlights
Retention is considered the second highest trial methods priority in the UK after recruitment.[1]
The main changes are: relative risk data are presented alongside absolute risks; the total number of women who attended the host trial primary outcome visit has been given in Table 1; we have added reference to Study Within A Trial (SWAT) 77 and SWAT 86 in our Discussion; we have added some context to the Discussion around the cost of the SWAT intervention; minor text clarifications throughout
A Study Within A Trial (SWAT) in a trial involving women aged between 70 and 84 years at high risk of osteoporotic fractures did find that sending a newsletter to participants approximately six weeks before a trial questionnaire increased retention by 1.6% (P = 0.05).[5]
Summary
Retention is considered the second highest trial methods priority in the UK after recruitment.[1] A recent UK study found that the median loss-to-follow-up in a sample of 151 trials was 11%.2. This reduces the amount of trial data available for analysis, which is especially problematic for the trial's most important outcome - the primary outcome - because this is the outcome that will be used to judge whether the trial intervention is effective.
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