Disclosed herein are methods that aid in the diagnosis and evaluation of a human subject that has sustained or may have sustained an injury to the head, such as mild or moderate to severe traumatic brain injury (TBI), using an early biomarker, ubiquitin carboxy-terminal hydrolase L1 (UCH-L1). Also disclosed here are methods that aid in determining whether a human subject that has sustained an injury or may have sustained to the head would benefit from and thus receive a head computerized tomography (CT) scan based on the levels of UCH-L1. These methods involve detecting levels and changes in levels of UCH-L1 in one or more samples taken from a human subject at time points within 24 hours after the subject has sustained or may have sustained an injury to the head.