Capacity refers to a person's ability or power to understand things and take actions in their everyday lives. A person is thought to possess this ability unless they have been assessed and deemed to lack capacity for a particular decision at a particular time.

According to the Mental Capacity Act 2005, a person lacks capacity if at the time a particular decision or action needs to be made, they are found to have an 'impairment of, or a disturbance in the functioning of, the mind or brain' that is sufficient enough to affect their capacity to make or communicate their decision.

When a person is found to lack capacity, it relates to their ability to make a specific decision at a specific time (e.g. whether or not to accept a named medical treatment at nine o'clock on a Tuesday morning) and their capacity to make other decisions (e.g. what to eat) may not be affected.

Why adults might lack mental capacity

Examples of an impairment or disturbance in the functioning of the mind or brain that might indicate a lack of mental capacity can include:

• Conditions associated with some forms of mental illness
• Illness related to age, such as dementia
• Significant learning disabilities
• Long-term effects of brain damage
• A physical or medical condition that can cause confusion, drowsiness or loss of consciousness
• Delirium
• Concussion following a head injury
• Symptoms of alcohol or drug use.

Healthcare professionals must not assume lack of capacity based on a patient's diagnosis and the patient's physical health must be taken into account. For example, a urinary tract infection could temporarily affect a patient's mental state.