14-15: Mild
9-13: Moderate
3-8: Severe
Workup is dependent on GCS severity, see:
Mild traumatic brain injury (GCS 14-15)
Clinical decision rules for<unk>  CT in trauma
Moderate-to-severe traumatic brain injury  (GCS <14)
Clinical decision rules for<unk>  CT in trauma
Head trauma 
Traumatic brain injury
Blast injury
Intracranial hemorrhage
Traumatic asphyxia
Moderate-to-severe traumatic brain injury
Trauma 
Elevated intracranial pressure