Decision making ability can change with time, depending on mental or physical health. Little is known about the factors that determine this change and the relationship of capacity to time. As a pilot for studies using functional mental capacities as an outcome measure, we sought to quantify this relationship measuring change over time using competence assessment tools, and rating scales for symptoms and global function. We assessed 37 inpatients in a secure psychiatric hospital. All patients met the diagnostic and statistical manual of mental disorders-fourth edition and International classification of diseases, 10th edition criteria for an Axis 1 mental illness, all with psychosis. Patients were interviewed twice a mean of 323 days apart (median 176 days range 17–1221 days). The MacArthur competence assessment tools for consent to treatment (MacCAT-T) and fitness to plead (MacCAT-FP) were used to quantify functional capacity along with the Positive and Negative Syndrome Scale (PANSS) and global assessment of function (GAF) scale. A comparison was also made between those patients prescribed clozapine in comparison to other antipsychotics. The number judged by treating psychiatrists to lack capacity either to make a treatment choice or to plead in court fell from 35 to 8 %. Change was greatest for those admitted within the previous 9 months. The measures of capacity improved between time 1 and time 2 for both consent to treatment and fitness to plead. The measures of capacity improved with positive symptoms within the PANSS and with GAF scores. Those with shorter lengths of stay at baseline had the greatest improvements in the MacCAT-FP scores. Effect sizes were medium or large (0.3–0.7+). As expected, patients prescribed clozapine had larger changes in functional mental capacities and larger effect sizes than those prescribed other psychotropics. The results show a strong relationship between the clinicians’ assessment of capacity and structured rating scales. We have shown that there is an improvement in capacity scores with time. More research is needed to compare the effect of treatment on capacity at fixed time intervals. It would also be helpful to look at a more general patient population.
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