The multiple facets of registry-based randomised controlled trials.

Traditional randomised controlled trials (RCTs) have long been considered the gold standard for evaluating clinical interventions. However, registry-based trials (RBTs) are emerging as a promising methodology, offering the potential for generating high-quality clinical evidence with greater external validity, reduced complexity, and lower costs. Despite being hailed as the "next disruptive technology in clinical research", the adoption of RBTs has been slower than anticipated. This lag could be attributed, in part, to the lack of consistent definitions for RBTs, which are often conflated with registry-based RCTs (RRCTs). Our analysis of seven RRCT reviews revealed significant variability in how RRCTs are defined and reported across clinical trial registries, reflecting the absence of standardised descriptors and nomenclature when registering these trials on clinical trial registries. This ambiguity complicates accurate estimation of their implementation and impact, reflected in reviews reporting widely varying numbers of trials registered as RRCTs. While RRCTs represent a valuable approach for addressing critical research questions, for their full potential to be realised, clear and consistent definitions, along with consensus on standardised registration terms, are essential. Given this, we would propose a taxonomy for RRCTs based on how the registry is used to support a RCT. We also welcome the CONSORT extension for the reporting of RRCTs and advocate for a standardised approach on how they are registered on clinical trial registries. Achieving this would not only enhance the recognition of RRCTs but also their impact on clinical practice. We propose a two-tier RRCT classification based on the number of outcomes captured in the registry and its use for identification or recruitment of participants, and encourage ongoing discussion around RRCT taxonomy to help guide future research.