39.2 The Mental Capacity Act 2005

3. The Mental Capacity Act 2005 (MCA)

The Mental Capacity Act 2005 (MCA) is the primary statutory framework in England and Wales governing decision-making on behalf of individuals aged 16 and over who lack mental capacity to make decisions for themselves.

The Five Statutory Principles of the MCA 2005

Section 1 of the MCA enshrines five core statutory principles that must guide every clinical interaction:

The Five Statutory Principles of the Mental Capacity Act 2005
  │
  ├── Principle 1: Presumption of Capacity
  │     └── Every person must be assumed to have capacity unless established otherwise
  │
  ├── Principle 2: Supported Decision-Making
  │     └── A person cannot be treated as lacking capacity unless all practicable steps to help them make the
  │         decision have been taken without success (communication aids, timing, family presence)
  │
  ├── Principle 3: Unwise Decisions
  │     └── A person is not to be treated as lacking capacity merely because they make an unwise or eccentric decision
  │
  ├── Principle 4: Best Interests
  │     └── Any act done, or decision made, on behalf of a person who lacks capacity must be in their best interests
  │
  └── Principle 5: Least Restrictive Option
        └── Regard must be had to whether the purpose can be achieved in a way that is less restrictive of the
            person's basic rights and freedom of action

The Two-Stage Functional Test of Capacity

Capacity is decision-specific and time-specific. A patient may possess capacity to consent to a simple dental examination or supragingival scaling, but lack capacity to make a complex decision regarding surgical crown lengthening or multiple extractions under general anesthesia. Furthermore, capacity may fluctuate dynamically over time (e.g., delirium, acute intoxication, transient ischemic attack, or sundowning in dementia).

Under Section 2 and Section 3 of the MCA, capacity must be evaluated using the Two-Stage Functional Test:

The Two-Stage Functional Test of Capacity (MCA 2005)

  Stage 1: The Diagnostic Test (Section 2)
    └── Is there an impairment of, or disturbance in the functioning of, the patient's mind or brain?
        (Permanent: Advanced dementia, severe learning disability, brain injury)
        (Temporary: Delirium, acute intoxication, severe sepsis, panic attack)
              │
              ├── NO ──> Patient Has Legal Capacity (Even if decision appears clinically unwise)
              │
              └── YES ─> Proceed to Stage 2
                           │
  Stage 2: The Functional Test (Section 3)
    └── Does that impairment prevent the patient from performing ANY ONE of the 4 statutory abilities?
              │
              ├── 1. Understand information relevant to the decision (nature, purpose, risks, alternatives)
              ├── 2. Retain that information long enough to make the decision
              ├── 3. Weigh or use that information as part of the decision-making process
              └── 4. Communicate the decision (speech, signing, blinking, written, or any other method)
              │
              ├── CANNOT perform 1 or more ──> Patient Lacks Capacity for this Specific Decision
              └── CAN perform all 4 ─────────> Patient Has Legal Capacity

[!NOTE] The "Unwise Decision" Rule: Under MCA Principle 3, a fully competent adult patient has the legal right to make an "unwise" or irrational decision (e.g., refusing endodontic therapy and insisting on extracting a restorable tooth, or refusing an urgent extraction despite a draining abscess), provided they understand the consequences of their decision. Clinicians cannot declare a patient incapacitated simply because they disagree with the proposed dental treatment plan.

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Quick Reference: The MCA at a Glance

• Capacity is decision-specific: a patient may have capacity for an examination but not for a general anaesthetic.
• Capacity is time-specific: delirium, intoxication and sundowning all fluctuate, so reassess rather than record a permanent label.
• The assessment, the steps taken to support it and the conclusion must all be recorded contemporaneously.

Jurisdiction and the Devolved Picture

Candidates must know that the Mental Capacity Act 2005 applies in England and Wales. Scotland is governed by the Adults with Incapacity (Scotland) Act 2000, which uses a different mechanism — a certificate of incapacity under section 47 completed by a medical practitioner authorises treatment, and there are Scottish equivalents of the attorney and guardianship provisions. Northern Ireland has the Mental Capacity Act (Northern Ireland) 2016. An SBA that specifies a Scottish patient is testing whether the candidate reaches for the right statute, and reciting the MCA 2005 for a patient in Glasgow is a wrong answer.

Applying the Functional Test in the Surgery

The functional test in section 3 asks whether the person can do all four of the following: understand the information relevant to the decision, retain it long enough to make the decision, use or weigh it as part of the process, and communicate the decision by any means. Failure of any one limb means capacity is absent for that decision at that time. The examinable subtleties are that retention need only be momentary — a person who can hold the information long enough to decide has capacity even if they forget afterwards — and that the inability must be caused by the impairment identified at stage one, which is why the diagnostic and functional stages are linked rather than independent.

Supporting the decision comes before concluding that capacity is absent. Practicable steps include using simple language and short sentences, easy-read materials with pictures, models and photographs, involving a familiar carer, an interpreter or a speech and language therapist, choosing the time of day when the person is at their best, holding the discussion in a familiar environment, and offering a second appointment rather than deciding on the spot.

Advance Decisions and Their Limits

An advance decision to refuse treatment made when the person had capacity is legally binding on clinicians if it is valid and applicable to the situation that has arisen. To refuse life-sustaining treatment it must additionally be in writing, signed by the person and by a witness, and include an express statement that it applies even if life is at risk. An advance decision cannot demand a particular treatment — it can only refuse — and it is overridden by a later, valid lasting power of attorney for health and welfare that covers the same decision. An advance statement of wishes and preferences is not binding but must be taken into account in any best interests decision.

Deprivation of Liberty and Restraint

Restraint under the MCA is lawful only if the person reasonably believes it is necessary to prevent harm to the incapacitated person and the restraint is a proportionate response to the likelihood and seriousness of that harm. Anything amounting to a deprivation of liberty requires separate authorisation under the Deprivation of Liberty Safeguards or a Court of Protection order. For dentistry this most often arises around treatment under general anaesthesia or sedation for an adult with a severe learning disability, and the examinable answer is that it requires multidisciplinary planning, documented best interests decision-making and, where necessary, formal authorisation — not a decision taken by the dentist alone on the day.