How should retrospectively registered protocols be interpreted in systematic reviews?

Prospective protocol registration is widely regarded as the preferred standard in systematic reviews. It promotes transparency from the outset of the research process, supports the prespecification of methods, contributes to reducing the risk of selective reporting and unnecessary duplication of effort, and is associated with improved methodological quality and reporting standards. However, despite these well-recognized advantages, not all systematic reviews are prospectively registered. In practice, retrospectively registered protocols continue to occur despite widespread recommendations for prospective registration. This raises an important question: how should retrospectively registered protocols be interpreted in systematic reviews? Within this context, the present commentary does not seek to position retrospective registration as equivalent to prospective registration. Rather, it aims to examine how retrospectively registered protocols may be interpreted in a transparent and methodologically informed manner, acknowledging that retrospective registration alone cannot demonstrate that methodological decisions were prespecified before the review process commenced. We propose that retrospectively registered protocols may be more appropriately interpreted through a transparency-oriented appraisal rather than a strictly timing-based judgment. Such an appraisal may consider whether the retrospective nature of registration is clearly reported, whether the reasons for delayed registration are explicitly described, whether deviations from the initial plan are transparently documented and justified, and whether overall methodological rigor and completeness of reporting are preserved in accordance with established guidance. Retrospective registration has inherent limitations, particularly its inability to prevent undisclosed methodological changes or reduce duplication of effort. Nevertheless, when clearly labeled, sufficiently detailed, and transparently reported, it may still contribute to the interpretability of the review process. Emphasizing transparency, consistency between planned and reported methods, and clarity of methodological decisions may support more robust critical appraisal and informed editorial decision-making in systematic reviews.