A guidance was developed to identify participants with missing outcome data in randomized controlled trials.

Background And Objectives: In order for authors of systematic reviews to address missing data in randomized controlled trials (RCTs), they need to first identify the number of trial participants with missing data. The objective of this study was to provide guidance for authors of systematic reviews on how to identify participants with missing outcome data in reports of RCTs.

Methods: Guidance statements were informed by a review of studies addressing the topic of missing data and an iterative process of feedback and refinement, through meetings involving experts in health research methodology and authors of systematic reviews.

Results: The proposed guidance includes (1) definitions of key terms, (2) 19 categories of participants described in RCT reports and who might have missing data, and (3) a flowchart on how to judge the outcome data missingness for each category. The judgment of missingness relies on how trial authors report on the categories and handle them in their analyses. Practically, for their primary analysis, systematic review authors should choose how to identify participants with missing outcome data (i.e., use either "definitely missing data" or "total possible missing data"), then select a method for handling missing data in meta-analysis. Sensitivity analyses should be undertaken to explore consistency with competing options for classifying patients as having missing data.

Conclusion: Adopting the proposed guidance will help promote transparency and consistency regarding how missing data are managed in systematic reviews.