• Athanasios Gkekas, Sarah Ronaldson, Adwoa Parker, and David Torgerson.
• Department of Health Sciences, York Trials Unit, University of York, York, UK.
• J Eval Clin Pract. 2024 Oct 22.

RationaleLoss to follow-up of participants can compromise the statistical validity of randomised trials. Moreover, it can have financial consequences for trial teams and funders. This study explores the Occupational Therapist Intervention Study (OTIS) where, despite a withdrawal rate of less than 10%, the trial team incurred opportunity costs related to participants who were initially recruited but subsequently decided to withdraw from the trial.Aims And ObjectivesTo estimate the cost of participant losses to follow-up in the OTIS trial and thus introduce a costing framework to research teams on how they could estimate the opportunity costs of participant withdrawal from their randomised trials.MethodsThe participants lost to follow-up are differentiated by (1) the time point at which they were lost to follow-up; (2) the treatment group they were allocated to; (3) their response patters to follow-up questionnaires; these elements were considered to identify the relevant types of attrition. Protocol-driven costs of trial materials, including administration, print, and shipping, were gathered. We calculated unit costs for each type of attrition by multiplying protocol-driven and intervention costs with the relevant number of participants. Summing up unit costs by type of loss to follow-up yields aggregate figures, enabling the estimation of aggregate and average opportunity costs of attrition.ResultsThe average cost per participant loss to follow-up in the OTIS trial is £98.41. The aggregate cost of participant loss to follow-up is £10,234.90 from the economic perspective of the trial team. Therefore, 1.42% of the allocated funding has been misallocated because of participant loss to follow-up.ConclusionDespite the low attrition rate of the OTIS trial, loss to follow-up has still generated considerable opportunity costs. It is recommended that decision makers focus on identifying strategies which could improve participant retention in randomised trials to optimise their budget.© 2024 The Author(s). Journal of Evaluation in Clinical Practice published by John Wiley & Sons Ltd.

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