SWAT 76 evaluation: randomised evaluation of sending pre-notification cards to trial participants before a face-to-face primary outcome measurement to increase attendance [version 2; peer review: 2 approved]

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