1. A method of determining the presence of traumatic brain injury (TBI) in a subject, comprising: collecting a first biological sample of cerebral spinal fluid (CSF) or blood from an injured subject suspected of having TBI within and up to 84 hours following occurrence of a suspected TBI; measuring said sample for an amount of neurensin (p24) in said sample; and comparing the amount of neurensin (p24) in said sample with the amount of neurensin (p24) in a respective sample from an uninjured subject; wherein an increased amount of neurensin (p24) in the subject sample relative to the amount in a CSF or blood sample from an uninjured subject is indicative of TBI in the injured subject.