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.