When to conduct a new systematic review or continue updating an existing living systematic review: rationale and considerations.

BACKGROUND: Living systematic reviews (LSRs) aim to synthesize all available evidence meeting pre-specified eligibility criteria to answer a research question and to continually update the review by incorporating new evidence as it becomes available. Through the initial version of an LSR is published, researchers determine whether a full update should be triggered. Over periodic iterations, modifications may be needed. Emerging evidence, external factors, methodological developments, and other considerations may necessitate adjustments to the original eligibility criteria, scope, or methods. However, guidance is limited on when cumulative changes warrant a new review rather than a continued update. DISCUSSION: In this paper, we provide rationale and guidance for deciding when updating an existing LSR is appropriate and when initiating a new, separate review is more appropriate. Several considerations should be considered, including changes in scope, eligibility criteria, or research question, reformulation of the conceptual framework, major methodological overhauls, foundational concerns with the original review, and practical, logistical, or publication considerations. In some circumstances, a hybrid approach involving continual updating of an existing review alongside development of a complementary new review may also be appropriate. Clear guidance on when to continue, restart, or branch into a new review will support the integrity, transparency, and usability of living evidence synthesis.