Assessing fidelity and adherence can be complex and vary according to the intervention/comparator (eg, one-off, short term repeated, long term repeated). Various deviations to the protocol can occur. For example, participants might initiate the intervention but then discontinue the intervention completely and permanently after a specific period of time, discontinue the intervention temporarily, reduce the dose, or modify the schedule. If relevant, authors should provide the prespecified definition for classifying participants as being treated as planned or not.

In addition, authors should indicate whether criteria were used to guide intervention/comparator modifications and discontinuations and where applicable describe these criteria. This information could be particularly important to evaluate the risk of bias due to deviations from the intended interventions, [210, 211] an important domain of the risk-of-bias tool developed by Cochrane [210]. Assessing this domain requires a clear understanding of, and ability to distinguish between, deviations that occur as planned in the protocol and deviations that arise due to the experimental context.

The research question (ie, explanatory v pragmatic) will affect the standardisation of the intervention/comparator as well as how adherence or fidelity is assessed or enhanced. In explanatory trials, the aim is to estimate treatment effect under ideal circumstances. The intervention/comparator are usually highly standardised with close monitoring of fidelity and adherence to the intervention/comparator and strategies to increase them. In contrast, pragmatic trials aim to determine treatment effect in clinical conditions. The intervention and comparator are usually highly flexible, and measurement of fidelity and adherence are unobstructive with no strategies to maintain or improve them. Nevertheless, assessing fidelity and adherence to the intervention/comparator, or at least recording the most important components of the intervention/comparator, is necessary to understand what was actually administered to participants. This is particularly important for complex interventions where diversity in the implementation of the intervention is expected. For example, in a pragmatic trial assessing a surgical procedure where the procedure is left to surgeons’ choice, investigators should plan to systematically record key elements related to pre-operative care, anaesthesia, the surgical approach, and post-operative care. This information is essential to provide a relevant description of the intervention that was actually provided when the trial is completed.

If the control group or intervention group received a combination of interventions, the authors should provide a thorough description of each intervention, an explanation of the order in which the combination of interventions were planned to be introduced or withdrawn, and the triggers for their introduction if applicable. Some complex interventions will require the development of specific documentation (eg, training materials, intervention manuals). Authors should make these available and indicate where they can be accessed.

If the control group is to receive usual care, it is important to describe what that constitutes so that readers can assess whether the comparator differs substantially from usual care in their own setting [212]. Various approaches could be used: standardising usual care to be in line with specific guidelines; or asking practitioners to treat control patients according to their own preference, which could result in heterogeneity of the care provided particularly between centres and over time [213]. Usual care can vary substantially across sites and patients, as well as over the duration of the trial. Further, it is important to clarify if the experimental group also received usual care in addition to the experimental intervention, and what actually differed between the groups. Usual care is frequently incompletely reported. In a review of 214 paediatric trials, the descriptions of standard of care were more often incomplete than the description of the intervention arms within the same study as measured by the TIDieR checklist (mean 5.81 (standard deviation (SD) 2.13) v 8.45 (SD 1.39)) [205].

If the control group is to receive a placebo, specific considerations need to be accounted for. Some evidence showed that placebos are insufficiently described [214]. Placebo could have several different aspects from pills to saline injections or more complex interventions such as sham interventions (eg, sham surgery) or attention control interventions. Authors should report the same level of details as required for the intervention—that is, content of the placebo or different component of the placebo, how and when it should be administered, material, procedure for tailoring the placebo to individual participants, and how fidelity and adherence were assessed or enhanced [207]. Complete reporting of the placebo is needed to understand what intervention effect is measured in the trial [215]. A network meta-analysis of osteoarthritis trials showed that different placebo interventions (oral, intra-articular, topical, oral and topical) had different effects and can impact the relative effect estimate of active treatments [216].

Further, the trial groups could receive different concomitant care in addition to the assigned trial interventions. Concomitant care could impact trial outcomes and bias effect estimates. To facilitate interpretation of study results and risk-of-bias assessments, authors should report relevant concomitant care that was allowed or prohibited where relevant.

Specific guidance has been developed to improve the reporting of interventions, particularly TIDieR, [23] TIDieR-Placebo for placebo and sham controls, [207] and the CONSORT extensions for non-pharmacological treatments [22]. Authors could consult these for more detailed information.