The most recent revision of the UKs Department of Health Code of Practice governing use of that procedure for the diagnosis of death reaffirms the preconditions for its consideration. These are:

There should be no doubt that the patients condition – deeply comatose, unresponsive and requiring artificial ventilation – is due to irreversible brain damage of known cause. There should be no evidence that this state is due to depressant drugs. Primary hypothermia as the cause of unconsciousness must have been excluded, and
Potentially reversible circulatory, metabolic and endocrine disturbances likewise. Potentially reversible causes of apnoea (dependence on the ventilator), such as muscle relaxants and cervical cord injury, must be excluded.With these pre-conditions satisfied, the definitive criteria are:

Fixed pupils which do not respond to sharp changes in the intensity of incident light. No corneal reflex. Absent oculovestibular reflexes – no eye movements following the slow injection of at least 50ml of ice-cold water into each ear in turn (the caloric reflex test). No response to supraorbital pressure. No cough reflex to bronchial stimulation or gagging response to pharyngeal stimulation. No observed respiratory effort in response to disconnection of the ventilator for long enough (typically 5 minutes) to ensure elevation of the arterial partial pressure of carbon dioxide to at least 6.0 kPa (6.5 kPa in patients with chronic carbon dioxide retention). Adequate oxygenation is ensured by pre-oxygenation and diffusion oxygenation during the disconnection (so the brainstem respiratory centre is not challenged by the ultimate, anoxic, drive stimulus).
Recognition of that state required the withdrawal of further artificial support so that death is allowed to occur, thus "sparing relatives from the further emotional trauma of sterile hope".In 1979, the Conference of Medical Royal Colleges promulgated its conclusion that identification of the state defined by those same criteria – then thought sufficient for a diagnosis of brain death – "means that the patient is dead". Death certification on those criteria has continued in the United Kingdom (where there is no statutory legal definition of death) since that time, particularly for organ transplantation purposes, although the conceptual basis for that use has changed. In 1995, after a review by a Working Group of the Royal College of Physicians of London, the Conference of Medical Royal Colleges formally adopted the "more correct" term for the syndrome, "brainstem death" – championed by Pallis in a set of 1982 articles in the British Medical Journal – and advanced a new definition of human death as the basis for equating this syndrome with the death of the person. The suggested new definition of death was the "irreversible loss of the capacity for consciousness, combined with irreversible loss of the capacity to breathe". It was stated that the irreversible cessation of brainstem function will produce this state and "therefore brainstem death is equivalent to the death of the individual". Diagnosis
In the UK, the formal rules for the diagnosis of brainstem death have undergone only minor modifications since they were first published in 1976.