Brain death is a medical diagnosis made by a physician.

Every hospital has a standardized set of criteria that it uses for
the determination of brain death and many hospitals also have a brain death committee that is charged with keeping up to date
on the latest medical information about brain death. Careful examination of the patient is required and all areas of the brain
are tested.

Any conditions that could mimic brain death, such as a severe drug overdose with barbiturates or extremely low body temperature, are ruled out.

In brain death, there is no spontaneous breathing. An apnea test
is often done to confirm this, taking the person off the ventilator until the carbon dioxide level in the blood reaches a level that would cause any living person to start to breathe.

Brain dead people do not exhibit reflexes that require brain function. For example, their pupils due not react to light by constricting, they do not gag if a tongue depressor is placed in
the back of their throat, and there is no response to pain.

Specialized medical tests may be done to further confirm brain death. This may include an electroencephalogram (EEG), which would show no electrical activity coming from the brain.

In brain death, there is no blood going to the brain at all; the brain's swelling prevents blood (and therefore oxygen) from reaching the brain. Two tests that may be used to demonstrate
the complete absence of blood flow to the brain include a
nuclear brain scan or a cerebral angiogram. During the brain
scan, a radioactive tracer is injected into a vein and the scanner
is used to make sure that there is no blood going to the brain.
In a cerebral angiogram, dye is injected into an artery and x-ray pictures are taken of the brain. Normally, there are four major arteries that supply a lot of blood to the brain; in a brain dead person, this x-ray of the blood vessels shows that no blood is going to the brain.

All of these tests are performed by doctors who specialize in the study of the brain and are explained to the patient's family, in addition to being carefully documented in the patient's medical chart.