More than expected? Assessing departures from additivity in health care utilization after mild traumatic brain injury in military personnel with pre-existing mental health conditions.

OBJECTIVE: Prior research indicates that there is an additive association between traumatic brain injury and mental health diagnoses on health-care utilization. This assumed additivity has not been formally assessed. The objective of this study was to estimate additive and multiplicative interactions associated with mild traumatic brain injury (mTBI) and pre-existing health conditions. METHOD: Active-duty military patient records over a nine-year period were sampled within four exposure groups (N = 4500 per group) defined jointly by incident mTBI and pre-existing mental health diagnoses. Outpatient and inpatient health encounters were compared between the four exposure groups using generalized linear models for count and proportion outcomes. Additive interactions were estimated using the interaction contrast ratio. Multiplicative interactions were estimated as a product term in the generalized linear models. RESULTS: The joint association of mTBI and pre-existing mental health diagnoses with health-care utilization, overall, was less than multiplicative and greater than additive. Patients with both exposures experienced more health-care utilization than expected under the assumed additivity (independence) of the two exposures. PTSD and anxiety diagnoses were the MH diagnoses associated with the largest interaction contrast values specific to total outpatient encounters. CONCLUSIONS: Studies of the interaction of two diagnoses on subsequent health-care utilization should examine both additive and multiplicative interactions. The greater-than-additive findings in this study indicate that there may be synergy, for at least some patients, between mTBI injury and mental health that complicates the treatment course.