Extension Reports in Clinical Trials: A Systematic Review and Consequent Guidance

Following the primary publication of a clinical trial, questions of longer-term efficacy and safety remain unanswered. Investigators often have sequel publications based on extended follow-up; yet, no guidelines exist for such extension reports.

The purpose of this analysis was to assess the characteristics of extension reports of cardiovascular trials published in 7 high-impact medical journals.

We searched MEDLINE to identify cardiovascular trial extension papers published between January 2019 and October 2023 in 7 high-impact medical journals. Extension reports were defined as those presenting additional follow-up data after publication of the primary results. Papers were screened by 2 reviewers before extracting data using a standard proforma.

After screening 1,388 papers, 69 extension reports across 64 trials were identified. A variety of cardiovascular topics were covered, coronary heart disease (n = 20) being the most common. Interventions were medical devices, drugs, surgery, or treatment strategies. Duration of follow-up varied considerably, most common being an original 1-year report (n = 20) with extension at 2, 3, 5, or 10 years. Nearly all extension reports had a superiority hypothesis (n = 68). Most extensions (n = 53) examined the same primary outcome as the original publication. There were also 7 nonrandomized open-label extensions, which suffered from the lack of a concurrent control arm. In 18 reports, treatment crossovers were permitted during the extension, and in 16 cases, alternatives to analysis by intention to treat were utilized. One-half of the extension studies were prespecified (n = 38), and only 10 had dedicated protocols or statistical analysis plans. No study used multiplicity corrections to control type I error across the totality of evidence. Landmark analyses were common (n = 39) which, while descriptively useful, were not based on the original randomized groups. Considerable loss to follow-up (>10%) occurred in 10 of 50 extension reports with clear reporting of this issue. For some trials, the conclusions regarding treatment effects were altered by the extended follow-up. In a previous survey of cardiovascular trials published in the New England Journal of Medicine, Lancet, and Journal of the American Medical Association in 2019, we identified a total of 23 of 84 trials involving extensions in some form.

This first systematic investigation into the reporting of extension studies found a great diversity in practice, noting several issues that need more adequate consideration. Considerations are provided on what trialists should consider when planning, conducting, and reporting extension studies.