Study design features associated with patient attrition in studies of traumatic brain injury: A systematic review

Loss to follow up or patient attrition is common in longitudinal studies of traumatic brain injury (TBI). Lack of understanding exists between the relation of study design and patient attrition. This review aimed to identify features of study design that are associated with attrition. We extended the analysis of a previous systematic review on missing data in 195 TBI studies using the Glasgow Outcome Scale (Extended) (GOS(E)) as an outcome measure. Studies which did not report attrition or had heterogeneous methodology were excluded, leaving 148 studies. Logistic regression found seven of the 14 design features studied to be associated with patient attrition. Four features were associated with an increase in attrition: greater follow-up frequency (OR: 1.2, 95% CI: 1.0-1.3), single rather than multi-center design (OR: 1.6, 95% CI: 1.2-2.2), enrolment of exclusively mild TBI patients (OR: 2.8, 95% CI: 1.6-4.9) and collection of the GOS by post or phone without face-to-face contact (OR: 1.6, 95% CI:1.1-2.4). Conversely, two features were associated with a reduction in attrition: recruitment in an acute care setting defined as the ward or intensive care unit (OR: 0.88, 95% CI: 0.47-0.72) and a greater duration of time between injury and follow-up (OR: 0.93, 95% CI: 0.88-0.99). This review highlights design features which are associated with attrition and could be considered when planning for patient retention. Further work is needed to establish the mechanisms between the observed associations and potential remedies