According to an embodiment, a method for diagnosing traumatic brain injury (TBI) in a subject may include repeatedly measuring heart rate variability (HRV) in the subject and a plurality of HRV altering variables; calculating an HRV fingerprint based on the subject's measured HRV and the measured plurality of HRV altering variables; generating a predicted HRV of the subject based on the HRV fingerprint; and diagnosing a TBI in the subject when the measured HRV of the subject deviates from the predicted HRV of the subject.