Age Suppresses the Association Between Traumatic Brain Injury Severity and Functional Outcomes: A Study Using the NIDILRR TBIMS Dataset.

OBJECTIVES Recovery from traumatic brain injury (TBI) is extremely difficult to predict, with TBI severity usually demonstrating weak predictive validity for functional or other outcomes. A possible explanation may lie in the statistical phenomenon called suppression, according to which a third variable masks the true association between predictor and outcome, making it appear weaker than it actually is. Age at injury is a strong candidate as a suppressor because of its well-established main and moderating effects on TBI outcomes. We tested age at injury as a possible suppressor in the predictive chain of effects between TBI severity and functional disability, up to 10 years post-TBI. SETTING Follow-up interviews were conducted during telephone interviews. PARTICIPANTS We used data from the 2020 NDILRR Model Systems National Dataset for 4 successive follow-up interviews: year 1 (n = 10,734), year 2 (n = 9174), year 5 (n = 6,201), and year 10 (n = 3027). DESIGN Successive cross-sectional multiple regression analyses. MAIN MEASURES Injury severity was operationalized using a categorical variable representing duration of posttrauma amnesia. The Glasgow Outcomes Scale-Extended (GOS-E) operationally defined functioning. Sociodemographic characteristics having significant bivariate correlations with GOS-E were included. RESULTS Entry of age at injury into the regression models significantly increases the association between TBI severity and functioning up to 10 years post-TBI. CONCLUSIONS Age at injury is a suppressor variable, masking the true effect of injury severity on functional outcomes. Identifying the mediators of this suppression effect is an important direction for TBI rehabilitation research.