Functional mental capacity is not independent of the severity of psychosis.

Background: Function-specific mental capacities are the legal criteria for competence. These are regarded as superior to clinical assessments of mental state and general function.

Aims: To determine whether tests of fitness to plead and capacity to consent are independent of each other and independent of mental state and global function in psychosis.

Method: The MacCAT-T and MacCAT-FP, PANSS and GAF were administered to 102 compulsorily detained forensic patients with psychosis. Criteria for incompetence were inability to express a preference concerning treatment, and independent rating as unfit to plead.

Results: MacCAT-T, MacCAT-FP totals and sub-scales correlated with each other and with PANSS and GAF. Those independently rated unfit to plead or who were incapable of making a treatment choice scored significantly worse on all rating scales. No test had satisfactory sensitivity or specificity.

Conclusions: Legal definitions of mind and of functional capacity offer a basis for structured clinical judgement regarding decision-making capacity. However, function-specific measures of understanding, reasoning and appreciation generate much the same results as measures of mental state and global functioning.