Objective: The capacity of persons with Alzheimer's disease or other neuropsychiatric disorders for giving consent to participate in research has come under increasing scrutiny. While instruments for measuring abilities related to capacity have been developed, how they should be used to categorize subjects as capable or incapable is not clear. A criterion validation study was carried out to help address this question.
Method: The authors measured the ability of 37 subjects with mild-to-moderate Alzheimer's disease and 15 elderly comparison subjects to provide consent for participation in a hypothetical clinical trial. Using the judgment of three experts as the criterion standard, the authors performed a receiver operator characteristic analysis for the capacity ability measures from the MacArthur Competence Assessment Tool-Clinical Research VERSION: The results were compared with categorizations of capacity status that were based on normative values.
Results: While most comparison subjects scored perfectly on all measures of the competence assessment tool, the majority of the group with Alzheimer's disease showed significant decision-making impairment. Thresholds based on normative values resulted in 84% (N=31) of the Alzheimer's disease subjects being rated as incapable on at least one ability; thresholds based on expert judgment resulted in 62% (N=23) failing to meet cutoff scores on at least one ability.
Conclusions: Even relatively mild Alzheimer's disease significantly impairs consent-giving capacity. But differentiating capable from incapable subjects remains an issue despite the aid of standardized tools. More research is needed to understand the relationship between subject factors (performance on ability measures) and categorical judgments about their capacity.