How do trial teams plan for retention during the design stage of the trial? A scoping review

Retention to trials is important to ensure the results of the trial are valid and reliable. The SPIRIT guidelines (18b) require "plans to promote participant retention and complete follow-up, including list of any outcome data to be collected for participants who discontinue or deviate from intervention protocols" be included in trial protocols. It is unknown how often protocols report this retention information. The purpose of our scoping review is to establish if, and how, trial teams report plans for retention during the design stage of the trial.

A scoping review with searches in key databases (PubMed, Scopus, EMBASE, CINAHL (EBSCO), and Web of Science from 2014 to 2019 inclusive) to identify randomised controlled trial protocols. We produced descriptive statistics on the characteristics of the trial protocols and also on those adhering to SPIRIT item 18b. A narrative synthesis of the retention strategies was also conducted.

Eight-hundred and twenty-four protocols met our inclusion criteria. RCTs (n = 722) and pilot and feasibility trial protocols (n = 102) reported using the SPIRIT guidelines during protocol development 35% and 34.3% of the time respectively. Of these protocols, only 9.5% and 11.4% respectively reported all aspects of SPIRIT item 18b "plans to promote participant retention and to complete follow-up, including list of any outcome data for participants who discontinue or deviate from intervention protocols". Of the RCT protocols, 36.8% included proactive "plans to promote participant retention" regardless of whether they reported using SPIRIT guidelines or not. Most protocols planned "combined strategies" (48.1%). Of these, the joint most commonly reported were "reminders and data collection location and method" and "reminders and monetary incentives". The most popular individual retention strategy was "reminders" (14.7%) followed by "monetary incentives- conditional" (10.2%). Of the pilot and feasibility protocols, 40.2% included proactive "plans to promote participant retention" with the use of "combined strategies" being most frequent (46.3%). The use of "monetary incentives - conditional" (22%) was the most popular individual reported retention strategy.

There is a lack of reporting of plans to promote participant retention in trial protocols. Proactive planning of retention strategies during the trial design stage is preferable to the reactive implementation of retention strategies. Prospective retention planning and clear communication in protocols may inform more suitable choice, costing and implementation of retention strategies and improve transparency in trial conduct.