BACKGROUND: Retaining participants in randomised controlled trials (RCTs) is challenging and trial teams are often required to use strategies to ensure retention or improve it. Other than monetary incentives, there is no requirement to disclose the use of retention strategies to the participant. Additionally, not all retention strategies are developed at the planning stage, i.e. post-funding during protocol development, but some protocols include strategies for participant retention as retention is considered and planned for early in the trial planning stage. It is yet unknown if these plans are communicated in the corresponding participant information leaflets (PILs). The purpose of our study was to determine if PILs communicate plans to promote participant retention and, if so, are these outlined in the corresponding trial protocol.
METHODS: Ninety-two adult PILs and their 90 corresponding protocols from Clinical Trial Units (CTUs) in the UK were analysed. Directed (deductive) content analysis was used to analyse the participant retention text from the PILs. Data were presented using a narrative summary and frequencies where appropriate.
RESULTS: Plans to promote participant retention were communicated in 81.5% (n = 75/92) of PILs. Fifty-seven percent (n = 43/75) of PILs communicated plans to use \"combined strategies\" to promote participant retention. The most common individual retention strategy was telling the participants that data collection for the trial would be scheduled during routine care visits (16%; n = 12/75 PILs). The importance of retention and the impact that missing or deleted data (deleting data collected prior to withdrawal) has on the ability to answer the research question were explained in 6.5% (n = 6/92) and 5.4% (n = 5/92) of PILs respectively. Out of the 59 PILs and 58 matching protocols that both communicated plans to use strategies to promote participant retention, 18.6% (n = 11/59) communicated the same information, the remaining 81.4% (n = 48/59) of PILs either only partially communicated (45.8%; n = 27/59) the same information or did not communicate the same information (35.6%; n = 21/59) as the protocol with regard to the retention strategy(ies).
CONCLUSIONS: Retention strategies are frequently communicated to potential trial participants in PILs; however, the information provided often differs from the content in the corresponding protocol. Participant retention considerations are best done at the planning stage of the trial and we encourage trial teams to be consistent in the communication of these strategies in both the protocol and PIL.

Freshwater shrimps in the family Atyidae exhibit two life history traits: 1) amphidromy, with many small embryos hatching into planktotrophic larvae that develop in the sea, and 2) landlocked, with few large embryos hatching into non-feeding lecithotrophic larvae, or facultative lecithotrophic larvae that develop in freshwater. The lecithotrophy of larvae is considered an adaptation to limited food conditions in the freshwater environment. Furthermore, faster development and settlement behaviours that large larvae exhibit are considered adaptations that allow the larvae to stay in or near parental habitats in fast-flowing streams. We therefore hypothesized that the facultative lecithotrophic larvae of landlocked shrimps might better adapt to limited food conditions as their large body size develops, an adaptation to maintain habitat position in flowing streams, than do planktotrophic larvae of companion amphidromous species developing in the sea. To test this hypothesis, we compared the larval feeding habits and size of two closely related species in the genus Paratya: the amphidromous P. compressa, with planktotrophic larvae, and the landlocked P. improvisa, with facultative lecithotrophic larvae. Larvae were reared by being fed commercially preserved or cultured phytoplankton (Tetraselmis sp.) and cultured zooplankton rotifers. Paratya compressa larvae did not survive, but P. improvisa larvae did develop into the juvenile stage under poor feeding conditions with preserved Tetraselmis alone, supporting our hypothesis of the effects of different larval feeding habits in amphidromous and landlocked atyid shrimp species. Hatchlings were larger and larval duration was shorter in P. improvisa than in P. compressa. Paratya improvisa larvae exhibited settlement behaviour beginning with the early zoeal stage. Our results also highlighted the retention strategy by which landlocked P. improvisa larvae stay in or near parental habitats.