Brain death is the complete and irreversible loss of brain function (including involuntary activity necessary to sustain life).
Brain death is one of the two ways of determination of death; the other way of determining death is irreversible cessation of circulatory and respiratory functions.
It differs from persistent vegetative state, in which some autonomic functions remain.

The determination of brain death can consist of four steps:

Prerequisites (all are required)
- Establish irreversible and proximate cause of coma. CNS (central nervous system) depressant drug effect absent (if indicated, toxicology screen; if barbiturates given, serum level <10 μg/mL). No evidence of residual paralytics (electrical stimulation if paralytics used). Absence of severe acid-base, electrolyte, endocrine abnormality.
- Achieve normothermia or mild hypothermia (core temperature >36°C). A warming blanket may be needed.
- Achieve a systolic blood pressure ≥100 mm Hg. Vasopressors or vasopressin are often required.
- Perform a neurologic examination.

Examination (required)
- Patient must lack all evidence of responsiveness: no eye opening or moving, no motor response.
- Absence of brainstem reflexes: pupils nonreactive to bright light; corneal reflex absent;
oculocephalic reflex absent (tested only if C-spine integrity ensured); no facial movement to noxious stimuli at supraorbital nerve, temporomandibular joint; gag reflex absent; cough reflex to tracheal suctioning absent; absence of motor response to noxious stimuli in all four limbs (spinally mediated reflexes are permissible).
- Apnea testing: absence of a breathing drive. Test: Ventilator adjusted to provide normocarbia (PaCO2 35–45 mm Hg); patient preoxygenated with 100% FiO2 for >10 minutes to PaO2 >200 mm Hg; patient well-oxygenated with a positive end-expiratory pressure (PEEP) of 5 cm of water; provide oxygen via a suction catheter to the level of the carina at 6 L/min or attach T-piece with continuous positive airway pressure (CPAP) at 10 cm H2O. Disconnect ventilator. Check for spontaneous respirations. Arterial blood gas drawn at 8–10 minutes, patient reconnected to ventilator; PCO2 ≥60 mm Hg, or 20 mm Hg rise from normal baseline value.
Or
- Apnea test aborted.

Ancillary testing (only one needs to be performed: to be ordered only if clinical examination cannot be fully performed due to patient factors, or if apnea testing inconclusive or aborted)
-Cerebral angiogram, electroencephalogram, transcranial Doppler, SPECT (single-photon emission computed tomography)

Documentation
- Time of brain death noted in medical record. Time is when the arterial PCO reaches the target value. In those with abnormal apnea test, time of death is when the ancillary test has been interpreted.

See also AAN Summary of Evidence-Based Guideline for Clinicians Update: Determining brain death in adults.

Adapted from American Academy of Neurology (AAN). Clinician Guideline Supplement: Practical Guidance. Update: Determining brain death in adults. June 2010. Internet. Accessed on May 27, 2016.