S-100B as Pre-Head CT Scan Screening Test After Mild Traumatic Brain Injury

Inclusion Criteria: * Patients presenting with head trauma to any of the AHCC/ERNES EDs, meeting the following criteria will be eligible to participate: * Presence of mild TBI per study definition * Arrival at the ED within 4 hours of injury * ED workup includes a Head CT (Controls are eligible if they present to the Outpatient Lab for routine bloodwork during enrollment hours) Exclusion Criteria: * Time of injury not able to be determined * Head CT not done as part of clinical emergency care * Incarcerated * Non-English or Spanish speakers * Absence of parent/guardian for minor subjects * Subject and/or parent/guardian do not have capacity to consent * Head injury due to birth trauma, anoxia, inflammatory, toxic, infectious or metabolic encephalopathies that are not complications of head trauma * Ischemic or hemorrhagic stroke without associated trauma (Controls are excluded if they have any of the following:History of brain tumor, melanoma and/or Alzheimer's disease; History of concussion, bone fracture or stroke (CVA or TIA) in the past month; History of surgery in the past month; Inability to obtain consent from subject and/or parent due to lack of capacity or absence of parent/guardian of minor; Inability to speak or read English; Incarceration)