Handling of missing outcome data in traumatic brain injury research: a systematic review

Traumatic brain injury (TBI) research commonly measures long-term functional outcome but studies often suffer from missing data as patients are lost to follow up. This review assesses the extent and handling of missing outcome data in the TBI literature, and provides a practical guide for future research. Relevant electronic databases were searched from January 1, 2012 to October 27, 2017 for TBI studies which used the Glasgow Outcome Scale or Glasgow Outcome Scale-Extended (GOS/GOSE) as an outcome measure. Studies were screened and data extracted in line with Cochrane guidance. A total of 195 studies, 21 interventional, 174 observational, with 104,688 patients were included. Using the reported follow-up rates in a mixed model, on average 91% of patients were predicted to return to follow up at 6 months post-injury, 84% at 1 year and 69% at 2 years. However, 36% of studies provided insufficient information to determine the number of subjects at each time point. Of 139 studies which did report missing outcome data, only 50% attempted to identify why data were missing, with just 4 reporting their assumption on the missingness mechanism. The handling of missing data was heterogeneous, with the most common method being its exclusion from analysis. These results confirm substantial variability in the standard of reporting and handling of missing outcome data in TBI research. We conclude that practical guidance is needed to facilitate meaningful and accurate study interpretation and therefore propose a framework for the handling of missing outcome data in future TBI research