Delirium (also known as acute confusional state) is an organically caused decline from a previous baseline of mental function that develops over a short period of time, typically hours to days. Delirium is a syndrome encompassing disturbances in attention, consciousness, and cognition. It may also involve other neurological deficits, such as psychomotor disturbances (e.g. hyperactive, hypoactive, or mixed), impaired sleep-wake cycle, emotional disturbances, and perceptual disturbances (e.g. hallucinations and delusions), although these features are not required for diagnosis. Delirium is caused by an acute organic process, which is a physically identifiable structural, functional, or chemical problem in the brain that may arise from a disease process outside the brain that nonetheless affects the brain. It may result from an underlying disease process (e.g. infection, hypoxia), side effect of a medication, withdrawal from drugs, over-consumption of alcohol, usage of hallucinogenic deliriants, or from any number of factors affecting ones overall health (e.g. malnutrition, pain, etc.). In contrast, fluctuations in mental status/function due to changes in primarily psychiatric processes or diseases (e.g. schizophrenia, bipolar disorder) do not, by definition, meet the criteria for delirium.Delirium may be difficult to diagnose without the proper establishment of a persons usual mental function. Without careful assessment and history, delirium can easily be confused with a number of psychiatric disorders or chronic organic brain syndromes because of many overlapping signs and symptoms in common with dementia, depression, psychosis, etc. Delirium may manifest from a baseline of existing mental illness, baseline intellectual disability, or dementia, without being due to any of these problems.
Diagnosis
Using the DSM-5 criteria for delirium as framework, the early recognition of signs/symptoms and a careful history, along with any of multiple clinical instruments, can help in making a diagnosis of delirium. A diagnosis of delirium cannot be made without a previous assessment of the patients baseline level of cognitive function. In other words, a mentally-disabled or impaired person might appear to be delirious, but may actually just be operating at their baseline mental ability. General setting
Multiple guidelines recommend that delirium should be diagnosed when it presents to healthcare services. Much evidence reveal, however, that delirium is greatly under-diagnosed. Higher rates of detection of delirium in general settings can be assisted by the use of validated delirium screening tools. Many such tools have been published. They differ in duration, complexity, need for training, etc. Examples of tools in use in clinical practice are:

Richmond Agitation and Sedation Scale (RASS) – highly sensitive and specific for diagnosing delirium in older patients
Observational Scale of Level of Arousal (OSLA) – highly sensitive and specific for diagnosing delirium in older patients
Confusion Assessment Method (CAM)
Delirium Observation Screening Scale (DOS)
Nursing Delirium Screening Scale (Nu-DESC)
Recognizing Acute Delirium As part of your Routine (RADAR)
4AT (4 As Test)
Delirium Diagnostic Tool-Provisional (DDT-Pro), also for subsyndromal delirium

Intensive care unit
People who are in the ICU are at greater risk of delirium and ICU delirium may lead to prolonged ventilation, longer stays in the hospital, increased stress on family and caregivers, and an increased chance of death.