Randomised controlled trials (RCTs) are conducted in accordance with a trial protocol. Having a protocol that is incomplete and non-transparent makes it difficult to critically appraise the trial [
1]. Protocols are needed for the readers of the corresponding paper to be able to fully appraise and interpret the results of the trial [
2]. As per the 2013 SPIRIT (Standard Protocol Items: Recommendations for Interventional Trials) definition, a protocol is “a document that provides sufficient detail to enable understanding of the background, rationale, objectives, study population, interventions, methods, statistical analyses, ethical considerations, dissemination plans, and administration of the trial, replication of key aspects of trial methods and conduct; and appraisal of the trial’s scientific and ethical rigour from ethics approval to dissemination of results” (3;202). Prior to the introduction of SPIRIT, the content and quality of protocols differed greatly [
3,
4], but the 33-item SPIRIT checklist for the minimum recommended protocol items improved this. Having a clearly written protocol increases transparency in trial conduct. Protocols are usually published prior to the trial paper publication [
5]. However, this is not always the case, and some protocol publications will require payment for access. For example, of the cancer clinical trials published in January of 2020 (
n = 113), only 11.3% had a publicly accessible protocol that was not behind a paywall [
2]. This further limits transparency and hinders replication in trial methods and conduct, which has been recommended for trial retention strategies in order to improve the evidence base for their effectiveness [
6].
Retention remains a major challenge for many clinical trials [
7]. The SPIRIT guidelines recommend the following information on retention be included in the trial protocol in Sect. 18b “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” (8;3). These of course are minimum requirements but the information can impact how missing data is dealt with and interpreted in the analysis of the trial [
8].
A recent Cochrane systematic review of strategies to improve retention in clinical trials found that there were no strategies for which the quality of evidence was high that showed improved retention. The review also highlighted that some trials implement multiple retention strategies [
6], and thus we questioned whether the use of multiple strategies is planned at the design stage and included in the protocol or are strategies implemented when retention becomes an issue within the trial. Evidence from a small number of interviews with trial staff is variable [
9]. Prospective retention planning informs appropriate costing and implementation of retention strategies but also increases transparency in trial conduct.