Doctors will assess the patient's condition by measuring vital signs and reflexes and by performing a neurological examination. They check the patient's temperature, blood pressure, pulse, breathing rate, and pupil size and response to light. They assess the patient's level of consciousness and neurological functioning using the Glasgow Coma Scale.

Imaging tests help in determining the diagnosis and prognosis of a TBI patient. Patients with mild to moderate injuries may receive skull and neck X-rays to check for bone fractures. For moderate to severe cases, the gold standard imaging test is a CT scan, which creates a series of cross-sectional X-ray images of the head and brain and can show bone fractures as well as the presence of hemorrhage, hematomas, contusions, brain tissue swelling, and tumors.

MRI may be used after the initial assessment and treatment of the TBI patient. MRI uses magnetic fields to detect subtle changes in brain tissue content and can show more detail than X-rays or CT. The use of CT and MRI is standard in brain injury treatment, but other imaging and diagnostic techniques that may be used to confirm a particular diagnosis include cerebral angiography, EEG, transcranial Doppler ultrasound, and SPECT.

Sometimes when the brain is injured, swelling occurs and fluids accumulate within the brain space. It is normal for bodily injuries to cause swelling and disruptions in fluid balance. When an injury occurs inside the skull-encased brain, however, there is no place for swollen tissues to expand and no adjoining tissues to absorb excess fluid. This leads to increased pressure is called intracranial pressure (ICP). High ICP can cause delicate brain tissue to be crushed, or parts of the brain to herniate across structures within the skull, causing severe damage.

Medical personnel measure a patient's ICP using a probe or catheter. The instrument is inserted through the skull to the subarachnoid level and is connected to a monitor that registers the patient's ICP. If a patient has high ICP, he or she may undergo a ventriculostomy, a procedure that drains cerebrospinal fluid (CSF) from the ventricles to bring the pressure down by way of an external ventricular drain (EVD).