Population estimates of undocumented incident traumatic brain injuries among combat-deployed US military personnel.

OBJECTIVE: To estimate the number of undocumented incident traumatic brain injuries (TBIs) among active component US military personnel serving in Iraq and Afghanistan prior to policy changes implemented in late 2006 and 2010 that improved TBI documentation.
METHODS: Negative binomial regression was used to model monthly incident TBI counts between December 2010 and June 2012 (N = 19) and then estimate expected monthly counts of incident TBIs during 2 periods: January 2003-October 2006 and November 2006-November 2010. Monthly amputation counts from Department of Defense surveillance data were used as a proxy for changing injury rates. Monthly active component deployment estimates derived from the Congressional Research Service, Brookings Institution, and Defense Manpower Data Center were used to estimate the size of the at-risk population each month. The difference between expected monthly incident TBI counts and reported counts is presented as the estimated number of undocumented incident TBIs.
RESULTS: The full model estimates that 21 257 active component military personnel experienced undocumented incident TBIs while deployed in Iraq or Afghanistan between January 2003 and October 2006, more than 4 times the 5272 incident TBIs documented during that period.
CONCLUSIONS: A sizeable majority of Iraq and Afghanistan combat veterans who experienced incident TBI while deployed prior to November 2006 are likely to have had their injuries undocumented, creating challenges for clinical care, disability evaluation, and future research.