In the UK and common law countries the legal test for decision-making capacity is not the test outlined by Zhong et al.Reference Zhong, Sisti and Karlawish¹ That test is derived from literature that emanates from the USA.

In common law jurisdictions, adults are presumed to have decision-making capacity, but this presumption can be rebutted for particular decisions if the person has some impairment or disturbance of mental functioning that renders him or her either: unable to comprehend and retain the information that is material to the decision, or; unable to use and weigh the information as part of the process of making the decision.² This common law test was codified into the Mental Capacity Act 2005 (UK) s3(1).

Contrary to Zhong et al’s rendering, the common law test does not incorporate an ability to ‘appreciate’ information. Indeed ‘appreciation’ was specifically rejected by the UK Law Reform Commission.³ To the extent that appreciation might be relevant it should be considered as part of the ability to comprehend. The ‘use and weigh’ arm of the common law test does not require that information be ‘rationally manipulate[d]’.Reference Zhong, Sisti and Karlawish¹ A competent person must have their decision respected even if his or her reasons are ‘irrational’.^2, Reference Ryan, Szmukler and Large⁴ Choices need not be ‘consistent’ over time, although if a person were to constantly change his or her mind that might be reason to question the usual presumption of decision-making capacity.^2, Reference Ryan, Callaghan and Peisah⁵ The bar for decisional ability does not rise as the risk of harm or complexity of the decision rises – it remains as described in the second paragraph above. However: as the risk increases, the more we should be concerned that the person has capacity, and; as the complexity increases, the more difficult it will be to attain the understanding of the relevant information required to demonstrate capacity.Reference Ryan, Callaghan and Peisah⁵

It is also worth highlighting that although the USA has not ratified the United Nations Convention on the Rights of Persons with Disabilities, almost all other countries, including the UK, have. Article 12 of the Convention places a duty on those who are assessing capacity to assist the person as much as possible to attain that capacity. This changes the process from one of objectively assessing the patient's abilities to one that determines whether the assessor can assist the patient to achieve those abilities.