The clinical manifestation of coma is the loss of the wakefulness-sleep cycle, and the patient is in a continuous deep sleep and cannot wake up. The patient’s perception, attention, thinking, emotion, orientation, judgment, memory and many other mental activities are all lost. The patient does not understand himself or the external environment and does not respond to external stimuli. The patient is unable to perform simple commands. When given a strong painful stimulus, there is no conscious response except for sometimes painful expressions or moans. What tests are required for patients with loss of consciousness? The complete diagnosis and differential points of coma should include 3 aspects: localization diagnosis, qualitative diagnosis and etiological diagnosis. 1. Localization diagnosis: Coma marks acute brain failure, which has a pattern of deterioration along the neural axis layer by layer. Generally, it is only necessary to monitor the brain function at the bedside to determine the plane of brain function impairment and the remaining functional plane of the comatose patient. 2. Qualitative diagnosis: It is mainly seen in extracranial systemic diseases, including most metabolic encephalopathies and toxic encephalopathies, but also in a few diffuse intracranial diseases, such as diffuse axonal injury, persistent status epilepticus, hypertensive encephalopathy and certain encephalitis. In the differential diagnosis, attention should be paid to past medical history, systemic examination and blood biochemistry and organ function tests.