Core Revision
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Chapter 1 of 11 · Ethics, Law & Safeguarding
Mental capacity: the two-stage test
Capacity is the gateway to every other decision in this guide. Get the order of the test right, and most AKT capacity questions answer themselves.
The Mental Capacity Act 2005 governs decision-making for people aged 16 and over in England and Wales who may be unable to decide for themselves. It is examined constantly, because it is one of the few areas where the law tells you exactly what to do and in what order — so the questions can be marked objectively.
The five statutory principles
Section 1 sets out five principles that apply to everything else in the Act. Learn them as a sequence, because AKT stems are usually testing whether you have skipped one.
| Principle (s1) | What it means on a Tuesday morning | |
|---|---|---|
| 1 | Presume capacity | A person aged 16 or over is assumed to have capacity until the contrary is proved. You need a reason to assess, not a reason not to. |
| 2 | All practicable steps first | Hearing aid in, interpreter booked, come back when the UTI is treated, use pictures. You cannot conclude incapacity until you have tried. |
| 3 | An unwise decision is not incapacity | The Jehovah’s Witness refusing blood, the smoker with COPD, the woman going home to a chaotic flat. Disagreeing with you is not a diagnosis. |
| 4 | Best interests | If they do lack capacity, anything done for them must be in their best interests. |
| 5 | Least restrictive option | Before acting, have regard to whether the purpose can be achieved as effectively in a way that interferes less with rights and freedom of action. Note the statutory wording — a duty to consider the less restrictive option, not an absolute duty to choose it. |
Principles 1–3 protect the person’s right to decide. Principles 4–5 constrain you once they cannot. A question that jumps straight to “what is in her best interests?” without establishing incapacity has usually failed principle 1 or 2.
The two-stage test
There are two stages and they must be done in this order.
Stage 1 — the diagnostic test (s2). Is there an impairment of, or a disturbance in the functioning of, the mind or brain? It can be permanent or temporary. It must not be established merely by reference to age, appearance, condition or behaviour.
Stage 2 — the functional test (s3). Is the person unable to do any one of the following in relation to this decision?
- Understand the information relevant to the decision
- Retain it — long enough to make the decision, no longer
- Use or weigh it as part of the process of deciding
- Communicate the decision, by any means at all
The crucial connector is the word because: the person must be unable to decide because of the impairment. Standard of proof: the balance of probabilities.
Failing one limb is enough
You do not need all four. A patient who understands and retains everything, but whose anorexia nervosa prevents them weighing the consequences of not eating, lacks capacity for that decision on the “use or weigh” limb alone.
A 78-year-old with moderate dementia is offered a hip replacement. You explain the risks; she repeats them back accurately. Two minutes later she has no recollection of them and asks the same questions again — and the cycle repeats every time you try to move on to comparing the options.
On the balance of probabilities, which limb of the functional test is she failing?
She understood — she repeated the risks back accurately. Understanding was demonstrated at the time. Look at what happens as soon as you try to move on.
She cannot retain the information long enough to make the decision. Note the statutory qualifier: retaining it “for a short period only” does not by itself mean she lacks capacity — the question is whether she can hold it long enough to use it. Here she cannot carry it from one moment to the next, so the decision cannot be made.
Practical step: try again with written information she can keep, and reassess. If retention is genuinely fleeting, move to a best-interests decision.
Nothing in the stem suggests she cannot weigh — she engaged with the risks while she held them. The failure is that she cannot keep hold of the information at all.
She is communicating clearly. The communication limb is for people who cannot express a decision by any means — a very small group, classically complete locked-in syndrome.
Pick an option to see the answer.
Decision-specific and time-specific
Capacity is never a global label. It attaches to one decision, at one moment. The same person may lack capacity to manage a property portfolio and have capacity to decide where to live, to consent to a flu jab, or to refuse an operation.
“The patient lacks capacity” is not a finding you can record. “Mr Ahmed lacks capacity to decide whether to have the amputation, assessed today” is. Any answer option that treats capacity as a permanent, whole-person attribute is wrong.
Fluctuating capacity
Delirium, hypoglycaemia, intoxication, early psychosis and the immediate aftermath of a seizure all cause capacity to come and go. The rule is simple: if the decision can wait, wait for a lucid interval and assess then. That is principle 2 — taking all practicable steps — in action. If it cannot wait, you act in best interests at the time, and you reassess.
A 54-year-old man attends with cellulitis. He is acutely intoxicated and refuses admission for IV antibiotics. He is not septic and there are no red flags.
What is the correct next step?
Assessing during acute intoxication, when the decision can safely be deferred, skips principle 2. You have not yet taken all practicable steps to help him decide.
The decision is not time-critical, so defer it. Intoxication is a classic temporary impairment: reassess when it has resolved and he may well have capacity to make his own choice — including an unwise one.
Safety-net clearly, document the plan and the reasoning, and give explicit return advice.
You are right not to override him, but simply walking away is not enough. He needs a documented plan, clear return advice, and reassessment once he is sober.
Relatives cannot consent for an adult in England and Wales unless they hold a registered health and welfare LPA or are a court-appointed deputy. Family are consulted in a best-interests decision, not asked to sign.
Pick an option to see the answer.
Who assesses, and when
The assessor is the person proposing the intervention — the decision-maker. For a flu vaccine in your surgery, that is you. For a hip replacement, the surgeon. There is no requirement for a psychiatrist, and no formal “capacity assessor” role. You may need specialist help for complex or finely balanced cases, but the responsibility does not transfer.
You assess when there is a reason to doubt capacity: an impairment is known or suspected, the decision is complex or serious, the person’s decision seems out of character, or they cannot explain their reasoning.
Two sections do the practical work once someone lacks capacity. Section 5 protects you from liability for acts done in connection with care or treatment, provided you took reasonable steps to establish capacity and reasonably believe the act is in the person’s best interests. Section 6 allows restraint only where it is necessary to prevent harm to the person and proportionate to the likelihood and seriousness of that harm — and it stops short of a deprivation of liberty, which needs its own authorisation (Chapter 3).
Scope note: the Mental Capacity Act covers England and Wales. Scotland uses the Adults with Incapacity (Scotland) Act 2000, which turns on benefit rather than best interests and authorises treatment by a s47 certificate. Northern Ireland has the Mental Capacity Act (Northern Ireland) 2016, only partly commenced.
Age 16 is the MCA threshold — the Act applies from 16, not 18. But you must be 18 to make an advance decision, to make an LPA, or to be deprived of liberty under DoLS. Three different ages, all commonly examined.
Self-testA 30-year-old with a learning disability needs a dental extraction. His mother says he cannot possibly consent. What is the first thing you should establish?
That there is a reason to doubt capacity for this specific decision — and then that you have taken all practicable steps to help him decide. A learning disability is a s2 impairment, but it does not tell you anything about s3. Many people with a learning disability have capacity to consent to a dental extraction when the information is given in accessible form, with pictures, in short sentences, with time.
His mother’s view is important evidence and she must be consulted if he does lack capacity — but she cannot make the finding, and she cannot consent for him.
Self-testYou assess a patient as lacking capacity to decide about a care home placement. She strongly objects and wants to go home. Does her objection matter?
Yes, in three ways. First, it is central to the best-interests checklist (s4) — her present wishes and feelings must be considered and given real weight. Second, it engages the least restrictive principle: could a package of care at home meet the need? Third, an objection is now expressly one of the factors in deciding whether an arrangement amounts to a deprivation of liberty (see Chapter 3).
What her objection does not do is reverse the capacity finding. Objecting is not the same as having capacity.
Self-testTrue or false: if a patient makes a decision that will clearly kill them, that is itself evidence they lack capacity.
False, and this is principle 3. A capacitous adult may refuse life-sustaining treatment for any reason, or no reason at all. The consequences of the decision are relevant only in so far as they show the person cannot use or weigh the information — not because the outcome is one you would not choose.
The distinction the examiners want: an unwise decision prompts a capacity assessment; it does not constitute one.
The MCA itself is unchanged, but the law on deprivation of liberty changed fundamentally on 2 June 2026, when the Supreme Court held that the “acid test” from Cheshire West was wrongly decided. Anything you have read on DoLS written before mid-2026 needs re-checking — Chapter 3 covers what replaced it.
Written and checked by NHS GPs. Last reviewed 24 August 2026. UK law and guidance change — always follow current local policy and your defence organisation’s advice in a real case.
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