Selection and reporting of usual care comparators when designing primary care trials of complex health interventions: a systematic review.

Background: Many primary care trials evaluating complex health interventions use a 'usual care' comparator. As 'usual care' can vary across clinical sites, countries, and over time, impacting trial design and raising ethical considerations attention should be given to its content prior to a trial starting.

Aim: To understand how researchers select and describe usual care comparators when designing primary care trials of complex health interventions.

Design and setting: A systematic review of primary care trial or feasibility study protocols.

Method: Electronic databases were searched from 1 July 2020 to 20 June 2022.

Results: A total of 83 protocols were included. A range of terms such as usual care and care as usual were used to describe usual care. The description of usual care varied significantly between protocols in terms of the level of detail provided regarding its selection and content. We categorised these descriptions according to the amount of detail they provided as: basic (72%), moderate (16%) and comprehensive (12%). Few protocols justified the content of their usual care comparator, with most simply commenting that it was based on clinical guidelines or current practice.

Conclusion: Different terms are used to describe usual care and most primary care researchers provide limited details on the section and content of their usual care comparators when publishing study protocols. This has implications for transparency and replicability, and suggests researchers continue to give limited attention to the content of usual care when designing their trials.