Mental capacity is the functional ability of a person to make decisions about healthcare, an attribute that reflects the central ethical principle of personal autonomy (Kerridge, Lowe, and Stewart 2013). Capacity forms the basis of the legal doctrines of consent to treatment (the process of gaining permission to treat) and informed consent (the duty to provide material information regarding the nature, purpose, benefits, and risks of having or not having treatment (see Reibl v Hughes  2 SCR 880; Rogers v Whitaker (1992) 175 CLR 479, 490; Montgomery v Lanarkshire Health Board  UKSC 1; Fitzpatrick v White  IESC 51).
In common law, all adults are presumed to have mental capacity, whereas children are presumed not to have mental capacity (Re T (adult: refusal of medical treatment)  4 All ER 649). These presumptions are rebuttable and may be overridden by evidence that an adult lacks capacity or that a child has capacity (Gillick v West Norfolk and Wisbech Area Health Authority  AC 112).
The common law test for mental capacity, which emerged from the case of Re C (Adult: Refusal of Treatment)  1 WLR 290 (see Box 1), requires a patient to be able to
understand treatment information,
retain the information,
weigh the information as part of a process of decision-making, and
communicate the decision to others.
Box 1 Case examples of mental capacity disputes
Re C (Adult: Refusal of Treatment)  1 WLR 290. In this English case, C was a 68 year old man with schizophrenia who was a long-term resident of what is now known as the Broadmoor Hospital. C developed gangrene of the foot and surgeons recommended a below knee amputation. C refused amputation on the basis he would rather die with two legs than live with one. C had a delusion that he was an internationally famous doctor. Despite this, Thorpe J found C competent to refuse treatment on the basis that he could comprehend and retain relevant information, believe it, and weigh it in the balance to arrive at a choice. Thorpe J found that the delusions did not impact on C’s ability to perform these tasks.|
Re MB  2 FCR 541. MB had consented to an emergency caesarean section, but she suffered needle phobia and resisted the administration of anaesthetics. Anaesthesia by mask was attempted but was unsuccessful. Further attempts to use a needle failed, even though MB continued to consent to the procedure. Doctors sought an order to permit them to forcibly provide the anaesthetic. The Court of Appeal of England and Wales found that MB’s phobia meant that she was “incapable of making a decision at all” and treatment was approved.
Brightwater Care Group Inc v Rossiter  WASC 229. In this Australian case, Rossiter was a quadriplegic who was dependent on artificial feeding and hydration through a feeding tube (PEG). Rossiter expressed a desire to refuse feeding and die. He gave numerous directions to carers to cease feeding and hydration. The Supreme Court of Western Australia found that it would be lawful to withdraw treatment as Rossiter’s capacity was attested to by several experts including a clinical neuropsychologist.
Auckland Area Health Board v Attorney-General  1 NZLR 235. A patient with Guillain-Barre Syndrome (a condition which impairs communication through progressive paralysis but does not impair cognition) was presumed to be incompetent by the court seemingly because of his inability to communicate
Fitzpatrick v K  IEHC 104. An African woman living in Ireland haemorrhaged after giving birth. After emergency treatment had commenced the staff were told by the patient that she was a Jehovah’s Witness and that she refused blood (on admission she had claimed to be a Roman Catholic). The patient suggested that she should be given Coca-Cola, tomatoes, eggs, and milk as an alternative to blood products. The treating obstetrician doubted the patient’s capacity to make a decision and approached the court for a determination on capacity and for an order authorizing treatment in the interim. These orders were made. The woman later appealed the decision. The court found that, while the patient may have been competent to refuse treatment, at the time there was sufficient uncertainty with regards to the patient’s ability to retain and understand information to justify the court order.
This test is part of the common law of Australia, Canada, Ireland, New Zealand, Singapore, the United Kingdom, and the United States. It also features in healthcare and guardianship legislation in all jurisdictions in Australia, apart from Western Australia (see for example the Medical Treatment Planning and Decisions Act 2016 (Vic), s 4), the provinces of Canada (see, for example, Adult Capacity and Decision-making Act, SNS 2017, c 4 s 3(d)), England and Wales (Mental Capacity Act 2005, ss 2, 3), Scotland (The Adults with Incapacity (Scotland) Act 2000, s 1(6)), Northern Ireland (Mental Capacity Act (Northern Ireland) 2016, s 4), Ireland (Assisted Decision-Making (Capacity) Act 2015, s 3(2)), New Zealand (Protection of Personal and Property Rights Act 1988, s 6), and Singapore (Mental Capacity Act 2008 (Chapter 177A), s 5).
More than ten years ago, many of these jurisdictions (including, Australia, Canada, Ireland, Singapore, and the United Kingdom) became signatories to the Convention of the Rights of Persons with Disabilities 2008 which, in Article 12(2), states that all people should be treated as enjoying “equal legal capacity.” Some commentators took this to mean that capacity testing should be abolished, but none of the signatories above have done so, and the standard legal capacity test has been retained in these jurisdictions (Stewart 2017).
Mental capacity should be assessed in adult patients with signs of impaired consciousness or cognition, including from head injury, drugs or alcohol, dementia, delirium, and mental illness (Biegler and Stewart 2001). Other relevant factors include high levels of pain and discomfort, shock, fatigue, panic, and fear. The potential for these factors to undermine the validity of consent means COVID-19 patients who present with them should have their capacity assessed.
A number of factors impacting on decision-making ability should be ameliorated prior to a capacity assessment. Emergency departments, for example, are often loud and stressful and so providing a quiet place, with family or support persons present, may help patients listen to and process information. Information should also be conveyed as clearly as possible (Jacob et al. 2005; UN General Assembly, Convention on the Rights of Persons with Disabilities A/RES/61/106, resolution adopted January 24, 2007). Visual aids can and should be used and translation services should be employed for non-English speaking patients.
A large number of capacity instruments, tools, and tests are available, although a variety of limitations exist which affect their clinical utility (Lamont, Jeon, and Chiarella 2013b). Importantly, tests such as the mini-mental status examination (MMSE) are designed to assess cognition and not capacity (Lamont, Jeon, and Chiarella 2013b). Cognitive tests may be useful screening tools—a lower MMSE score increases the chance a patient lacks capacity—but they are not diagnostic (Stewart and Biegler 2004).
Arguably, the most widely tested and used capacity assessment instrument is the Macarthur Competency Assessment Tool for Treatment (MacCAT-T) (Grisso and Appelbaum 1998). A recent study found that the MacCAT-T closely corresponded with the legal test of capacity (Curley et al. 2019). A difficulty with the MacCAT-T test is, however, that it takes around twenty minutes to complete, which may be impracticable in emergency and critical care settings. Evidently, an alternative method is needed.