The assessment of mental capacity.

Mental capacity is required for an adult to make autonomous treatment choices. This review highlights recent legal and clinical developments in the field. The recent English Mental Capacity Act 2005 is described and compared with mental health legislation. Some of the difficulties of defining mental capacity are then outlined. Recent research regarding mental capacity in general hospital patients is summarised. Such research indicates firstly that capacity can reliably be assessed; secondly, that among general hospital inpatients, approximately one-third may lack capacity; and thirdly, that mental incapacity in this setting is mainly driven by cognitive impairment caused by delirium or dementia. This is contrasted with psychiatric inpatients, where the problem is no more frequent, but mainly relates to psychotic illness. The article finishes with some general guidance as to how to assess mental capacity in the general hospital.