Studies have shown that assessment of a patient's capacity to provide informed consent is likely to be biased if the physician relies on a general impression of the patient's ability to understand the information. Mental status tests, such as the standardized Mini-Mental Status Examination (MMSE), are sometimes used to assess the patient's capacity to consent to treatment. Etchells and associates compared the reliability of three methods of evaluating the patient's capacity to provide informed consent: the standardized MMSE, a specifically designed semistructured capacity assessment and a capacity assessment conducted by experts in this type of evaluation.

The study included 100 patients who, at the time of entry into the study, were facing non-emergent decisions about treatment and were either refusing treatment or consenting to treatment but were not clearly capable of making an informed decision. Patients who were clearly capable to make an informed decision and were consenting to treatment were excluded from the study.

The authors developed a semistructured instrument, called the Aid to Capacity Evaluation (ACE), that prompts inquiry into seven relevant areas (see the accompanying table). After each area is assessed, the clinician decides whether the patient is “definitely incapable,” “probably incapable,” “probably capable” or “definitely capable.” Medical residents and senior medical students administered the ACE assessment.

In addition to the ACE, the standardized MMSE was performed, as well as an assessment of capacity by two independent experts. The results of these three measures were compared, and likelihood ratios (LR) were calculated for the ACE and the MMSE.

The expert evaluations revealed that 37 percent of the 100 patients were not capable of making an informed consent to treatment. If the ACE result was “definitely incapable,” the probability of incapacity was 92 percent (LR: 20). If the ACE result was “definitely capable,” the probability of incapacity dropped to 3 percent (LR: 0.05). If the ACE result was “probably incapable” or “probably capable,” the chance that a patient would be improperly classified according to the ACE was increased.

When a patient had a standardized MMSE score of zero to 16, the probability of incapacity was 89 percent (LR: 15), whereas a standardized MMSE score of 24 to 30 decreased the probability of incapacity to 3 percent (LR: 0.05). When an ACE result of “probably incapable” or “definitely incapable” was combined with a standardized MMSE score of zero to 16, the probability of incapacity increased to 96 percent (LR: 40). Similarly, when the ACE result showed that the patient was “probably capable” or “definitely capable” and the MMSE score was 24 to 30, the patient had only a 3 percent probability of incapacity (LR: 0.05).

The authors conclude that a specific instrument to assess capacity, such as the one they developed, can closely agree with an assessment made by an expert and can be administered by medical students, residents or physicians. The combination of the MMSE and a method of assessment like the ACE may provide information about a patient's capacity to consent to treatment.