Objective: (1) Examine systematic biases created by subjects lost at 1-year follow-up in samples of persons with traumatic brain injury; (2) identify potential threats to generalization of outcomes data.
Design: A consecutive sample of admissions to acute rehabilitation studied 1 year following discharge.
Setting: An inpatient brain injury rehabilitation unit in a large, academic medical center.
Subjects: Eighty-eight patients with primary diagnosis of traumatic brain injury.
Main outcome measures: Subjects were considered lost to follow-up when phone calls, mail, clinic visits, and assistance from family failed to allow contact 1 year after discharge from acute rehabilitation. Potential effects of the biased follow-up sample were examined for seven suboptimal outcomes.
Results: A total of 38.6% of subjects were lost to follow-up. Subjects intoxicated at time of injury and those with history of substance abuse were more-likely to be lost. Among subjects followed, the likelihood of working or being in school 1 year after discharge was significantly less for those intoxicated at time of injury and those with a history of substance abuse.
Conclusions: Systematic bias in longitudinal studies may result from subjects with substance use problems being lost to follow-up. Population estimates for return to work or school will be overestimated if those lost who have substance use problems resemble those followed.