Mental capacity to make decisions on treatment in people admitted to psychiatric hospitals: cross sectional study.

OBJECTIVE To estimate the prevalence of mental capacity to make decisions on treatment in people from different diagnostic and legal groups admitted to psychiatric hospital. DESIGN Cross sectional study. SETTING General adult acute psychiatric inpatient units. PARTICIPANTS 350 consecutive people admitted to psychiatric wards from the community over 16 months. MAIN OUTCOME MEASURE Mental capacity assessed by clinical interview and the MacArthur competence assessment tool for treatment. RESULTS Estimates of mental capacity were obtained on 97% (n=338) of the 350 people admitted. Of those an estimated 60% (95% confidence interval 55% to 65%) lacked mental capacity to make decisions on treatment. This proportion varied according to diagnosis, ranging from 97% (n=36) in people with mania to 4% (n=24) in people with personality disorder. Mental incapacity was common in patients admitted informally to the psychiatric wards (n=188; 39%, 32% to 46%). Incapacity and detention are closely associated under non-capacity based mental health law. CONCLUSIONS Mental incapacity to make decisions on treatment is common in people admitted to psychiatric wards from the community but cannot be presumed. It is usual in those detained under the Mental Health Act and common in those admitted voluntarily.