Utility and Limits of the Mini Mental State Examination in Evaluating Consent Capacity in Alzheimer's Disease

The study group comprised 37 persons. Sixty percent were women. The mean±SD age was 78.7±5.8 years, the mean number of years of education was 13.8±3.2, and the mean MMSE score was 22.9±3.8 (range, 16 to 28).

Table 1 lists the areas under the ROC curves and their 95 percent confidence intervals for the MMSE as a test for identifying incapacity against the three criterion standards established by the MacCAT-CR or the combined standard. None of the confidence intervals include .5—the expected area if the null hypothesis were true—indicating that the MMSE significantly adds to the identification of incapacity. However, this significant effect is modest: the areas are relatively small, and no cutoff point yields both high sensitivity and high specificity.

High specificity, in the range of 85 to 94 percent, was approached at MMSE scores of 19 or less. Such a cutoff score correctly identified 39 to 54 percent of the incapable subjects. High sensitivity, ranging from 91 to 100 percent, was approached at an MMSE score of 26 while a specificity of 29 to 36 percent was retained.

Past attempts to define the utility of the MMSE in capacity assessments have focused on treatment consent in heterogeneous patient groups, with mixed results (4). Our study is the first to assess the screening utility of the MMSE in a group with well-defined Alzheimer's disease, using a validated capacity instrument specifically designed for research on informed consent, the MacCAT-CR, as a benchmark.

Within a certain range of scores, the MMSE was at best a modest discriminator of decisional capacity in this study group. This range, 21 to 25, includes the cutoffs often used for "normal" for the MMSE. Given that abilities relevant to competence are correlated with executive functions (10), the modest discriminatory power of the MMSE may reflect the instrument's relative insensitivity in detecting executive dysfunction.

Do these findings imply that the MMSE has no role in screening for incapacity? The utility of a test depends partly on its purpose. For instance, for a nontherapeutic, greater-than-minimal-risk study, a researcher may have strong ethical reasons to select only patients who are clearly capable of providing consent. Such a researcher would be advised to use a higher MMSE cutoff score—for example, a cutoff of 26 has a sensitivity of 91 to 100 percent in identifying persons who are incapable of providing consent—as a prelude to a more comprehensive task-specific evaluation. Alternatively, if the goal were to identify as many capable subjects as possible, even at considerable expense of effort and time, then a cutoff of 19, which has a specificity range of 85 to 94 percent, might be used. Anyone who scores below that level is very likely to be deemed incapable of providing consent.

Thus, although the MMSE is not a very good predictor of incapacity, our findings suggest how best to extract the modest amount of utility the instrument offers—with virtually no additional effort, because the scores are already available for most patients with Alzheimer's disease.

Our study is limited by its small sample, the use of a hypothetical research scenario to assess capacity, and its time-slice view of complex phenomena that may fluctuate over time. Competency judgments also take into account important contextual issues, such as the risk-benefit calculus (1). In this study, we used a scenario for a placebo-controlled, double-blind, randomized trial for a new treatment for Alzheimer's disease. Thresholds for competence cannot be assumed to be equal across research studies with different risk-benefit ratios. Finally, because sensitivity and specificity are features of tests and not of populations, the true usefulness of the MMSE as a screening device will depend on the prior probability of incapacity in a particular situation.

This research was supported in part by grants MH-18911 and MH-64172 from the National Institute of Mental Health and by the American Association for Geriatric Psychiatry Eisai-Pfizer Alzheimer's Disease Research Fellowship awarded to Dr. Kim. The authors thank Colleen McCallum, M.S.W., Pierre Tariot, M.D., and Yeates Conwell, M.D.

The authors are affiliated with the department of psychiatry of the University of Rochester School of Medicine and Dentistry, Box PSYCH, 300 Crittenden Boulevard, Rochester, New York 14642 (e-mail, [email protected]). Dr. Kim is also director of the program in clinical ethics at the University of Rochester Medical Center.