A high-dependency unit (HDU) is an inpatient psychiatric ward, typically with only a small number of beds, attached to an acute admissions ward.^[1] It is intended to provide treatment for patients who cannot be managed safely on an acute (open) ward, but do not meet the threshold for admission to PICU.^[2][3] The number of staff per patient will be higher than the associated acute ward, but may be lower than levels at a PICU.

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According to the guidelines for the operation of mental health high-dependency units in Queensland Mental Health Act 2000, HDU can be defined as "a separate potentially lockable area within a mental health inpatient facility, designed to provide for the safe management of involuntary patients requiring a higher level of individual care."^[4]

In most hospitals, a high-dependency unit is a separate unit within the psychiatric ward for inpatients who require special care for mental illnesses and usually has an increased staff-to-patient ratio. People in HDUs need more intensive treatment, observation, nursing care, increased level of supervision and intervention in a safe environment. Patient spending time in HDU varies depending on the nature of their illnesses and demands.

Reasons for placing a person in an HDU varies and may include any of the following such as if someone poses significant harm or threat to others/self, chances of absconding which will likely lead to a deterioration in the person's mental and/or physical condition.^[5]