Impaired understanding in evaluating DMC in both medical and psychiatric patients

First, although it is tempting to emphasise the differences between the two patient groups' performance on the three criteria for DMC, we should begin with what they have in common. The authors found that in both hospital groups, about a quarter had poor understanding (defined as ⩽50% score on the understanding subscale); having poor understanding was a strong indicator of incapacity regardless of hospital group, with 81-93% lacking DMC. Those with poor understanding scores who lacked DMC generally also had poor appreciation and reasoning scores. This makes sense because if someone cannot absorb information then it is difficult to form beliefs about it or manipulate it (analogous to someone with impaired attention who appears to have poor short-term memory). Although it is not surprising that 67% of those with incapacity among medical patients had poor understanding (where one would expect cognitive dysfunction more than psychopathology to drive impairment), it is notable that almost half (44%) of the psychiatric patients lacking decisional capacity had poor understanding. This is consistent with previous studies of psychiatric patients (those with chronic psychoses) showing that cognitive impairment, rather than classic positive psychotic symptoms, can be the primary source of incapacity for many people in this patient group. ^{Reference Palmer and Savla5,Reference Carpenter, Gold, Lahti, Queern, Conley and Bartko6} At any rate, clinicians should remain aware that lack of understanding accounts for nearly half of cases of incapacity even among psychiatric patients.

What is the theoretical and empirical significance of impaired reasoning?

Second, the theory behind DMC says that understanding is not sufficient for intact capacity and the authors confirmed this: 39% of psychiatric patients who showed good understanding nevertheless lacked DMC - virtually all of them had poor scores on appreciation, whereas only 17% had poor scores on reasoning. These are presumably patients who, despite their intellectual comprehension and intact reasoning, are unable to form adequate beliefs about their illness or treatment - a pattern attributable to their psychopathology (such as delusions) more than to cognitive impairment.

In contrast, among the medically ill, few patients with good understanding lacked DMC (13%). Everyone (12 of 12) in this small group lacking DMC scored poorly on reasoning yet most (10 of 12) had good scores on the appreciation task. These patients who were apparently determined to be lacking in DMC solely on the basis of poor reasoning present somewhat of a puzzle because they do not fit neatly into our normative understanding of incapacity. A comprehensive legal review by the authors of the MacCAT instruments found that poor reasoning is not used by the courts as a sole basis for finding someone as incapable. ^{Reference Berg, Appelbaum and Grisso7} There are several potential explanations for this apparent discrepancy.

It may be a reflection of the fact that operationalisation of the reasoning ability in the MacCAT instrument is based on psychological, rather than legal, concepts, such that an expectation of close tracking between historical legal findings and clinical determinations using MacCAT may not be warranted. ^{Reference Kim2} Also, we do not know whether the clinicians themselves justified their determinations of these patients' status using the reasoning standard - perhaps, for instance, despite ‘good’ understanding scores, the clinicians focused on the patients' failure to understand or appreciate specific items of particular import or that a 50% threshold for a ‘good’ score is insufficiently sensitive to detect understanding impairment sufficient for incapacity (after all, the 50% cut-off is somewhat arbitrary and low). Or, finally, it could be that UK clinicians in fact do use reasoning as a standalone standard, unlike in US courts. An in-depth qualitative analysis of these patients' interviews may yield insights. It would be instructive to find out just what kind of reasoning deficiency is compatible with intact understanding and appreciation and yet serious enough to merit a determination of incapacity. In this way, an empirical analysis could inform our normative discussions.

Research challenges arising from the context-specific nature of DMC

Finally, this analysis provided an important opportunity to examine a key methodological question about ‘real-world’ DMC studies. This study had important strengths: the subjects were real patients and the DMC capacity assessed was for real treatment decisions (albeit not real time). Further, the authors had available well-validated categorical DMC judgments (which is surprisingly uncommon in DMC research), making it possible for them to assess the relationship between the dimensional aspect of individual abilities with a categorical outcome variable for DMC. The methodological question arises from the fact that DMC assessment must be tailored to a particular decision, taking into account key contextual factors. ^{Reference Grisso and Appelbaum1,Reference Kim2} Thus, some argue that the level of abilities needed to consent to psychiatric hospital admission should be lower than that needed for other decisions. ^{Reference Appelbaum, Appelbaum and Grisso8} Also, whether patients consent to or refuse a treatment raises different risk-benefit considerations. ^{Reference Grisso and Appelbaum1,Reference Kim2,Reference Buchanan and Brock9} Psychiatrists in fact use differential thresholds depending on the risk-benefit context of the decision. ^{Reference Kim, Caine, Swan and Appelbaum10} Thus, it is possible that a competent patient (agreeing to an in-patient admission) could actually have worse performance on ability measures than someone deemed incompetent (but refusing to undergo an operation that is minimally burdensome but lifesaving). This potential variability in the meaning of the main outcome variable could present a significant complication to ‘real-world’ DMC studies.

The results of the Owen et al analysis are reassuring in that decision-specific DMC judgments by clinicians, even for a variety of medical and psychiatric decisions, still allowed detection of important associations between levels of ability and categorical DMC status. It may be that the risk-benefit range of the decisions was not sufficiently wide enough to affect categorical judgments. Or perhaps patients with relatively intact capacity tended to agree with recommended treatment whereas the more impaired patients were more likely to refuse - in which case the associations between categorical status and dimensional performance scores would be accentuated. (This might also explain the remarkably high reliability of categorical judgments among the clinician evaluators in this study.) As we conduct more real-world studies using actual patients' categorical DMC status, this is an aspect of the modern theory of DMC that researchers should incorporate into their work by explicitly including, for example, the valence of the patient's choice (acceptance or refusal of recommended treatment) in the analysis.