Assessing mental capacity

Assessing a patient's capacity to consent is essential, and it’s important to take all practicable steps to help patients make decisions.

Assessing a patient's capacity

  • Assessing mental capacity is an integral part of being a doctor.
  • Understanding issues around a patient's ability to make a decision about treatment is central to the doctor-patient relationship.
  • It's important take all practicable steps to help a patient make the decision themselves.
  • Capacity is time and decision specific.

This guide examines the law and guidance in England and Wales.

In Scotland, the relevant legislation is the Adults with Incapacity (Scotland) Act 2000 and the relevant Code of Practice. In Northern Ireland, the Mental Capacity Act (Northern Ireland) 2016 received Royal Assent in May 2016 and will govern this area once fully in force. Members are welcome to contact us for further advice.

What is capacity?

Mental capacity is the ability to make a decision or take an action that impacts on a person's life. It means that a person is able to make a decision about their own care and treatment.

It's important not to assume incapacity based on someone's appearance, condition, or anything else. But if you have reason to believe that a patient may lack the mental capacity to make their own decision about a treatment or course of action you're recommending, you need to assess their mental capacity.

The Mental Capacity Act in England and Wales

Assessment of mental capacity is set out in the Mental Capacity Act (MCA) 2005 and its accompanying Code of Practice.

It provides a framework for caring for how to make decisions for those aged 16 or over in England and Wales who lack the ability to make decisions for themselves.

  • The MCA requires you to assume that every adult aged 16 or over has mental capacity to make their own decisions, unless proved otherwise.
  • If mental capacity is in doubt, you should assess a patient's capacity before treatment.
  • Capacity is time and decision specific. This means you should assess a patient's ability to make a specific decision at the time the decision needs to be made.
  • A person's inability to make a major or complex decision doesn't mean they can't make a smaller or simpler decision.

The MCA embodies five key principles. The first three help you determine whether a patient lacks capacity. If they do, the final two principles are used to support the decision-making process.

Principle 1: the presumption of capacity

  • Capacity should be assumed unless proved otherwise.
  • Don't assume someone is unable to make a decision based on their medical condition or disability.

Principle 2: support the individual

  • A patient should be given all practicable help before they are deemed unable to make their own decisions.
  • Make every effort to support a patient in making a specific decision for themselves if possible (eg, providing support through an advocate, translator, speech and language therapist etc.).
  • Even if you establish the patient lacks mental capacity, you should still involve them as far as possible in making their decision.

Principle 3: unwise decision

  • A patient has the right to make a decision that you might disagree with or consider irrational or unwise.
  • This doesn't indicate a lack of mental capacity, but may reflect individual preferences or values.
  • The ability to make the decision is key; not the decision itself.

Principle 4: best interests

  • Decisions made or action taken for or on behalf of a person who lacks mental capacity must be done in their best interests.
  • 'Best interests' isn't defined by the MCA or the Code of Practice and depends on individual circumstances. These include a person's welfare, social, emotional and psychological interests as well as their medical interests.
  • The rare exceptions to this principle are around research and advance refusals of treatment, where other safeguards apply.

Principle 5: least restrictive option

  • If you're making a decision about treatment for a patient lacking capacity, you must consider whether it's possible to delay until the person regains capacity.
  • If this isn't possible to defer the decision until the patient regains mental capacity, make the decision in the person's best interests, finding the least restrictive option wherever possible. This means the option that would interfere less with the patient's basic rights and freedoms.

Two-stage assessment of capacity

Determining whether someone has the capacity to make a particular decision involves a two-stage test.

  • Stage 1: whether there is an impairment of or disturbance in the functioning of the patient's mind or brain, whether temporary or permanent.

If so, then:

  • Stage 2: is the impairment or disturbance sufficient that, even with all practical and appropriate support, the patient is unable to make a particular decision at the time it needs to be made?


The MCA states that someone is unable to make a decision if they cannot do one or more of the following:

  • understand the information relevant to the decision (including the reasonably foreseeable consequences of whatever decision is made or of failing to make a decision)
  • retain that information in their mind
  • use or weigh the information as part of the decision-making process
  • communicate the decision by any means, including speech, sign language, or simple muscle movement.

A patient failing one or more aspects of this test is deemed to lack the relevant mental capacity, but you must try to find ways of communicating with a patient before determining they lack capacity.

Take practical steps to help and support the patient to make their own decision. Carefully explain all the information they need to decide on, presenting the information in ways best suited to the person. Those close to the patient, or carers, might be able to help you communicate with the patient or let you know the best time to communicate with them.

Best interests

If a patient lacks mental capacity but has not made a (valid and applicable) advance decision you need to work out their best interests.

Read more on assessing a patient's best interests when they cannot consent to treatment.


  • Record why there is uncertainty about the patient's mental capacity, as well as details of your assessment and conclusions.
  • The record should clearly show how any decision about the patient's best interests was reached, the reasons for the decision, who was consulted, and what factors were considered.

The MCA Code of Practice provides further guidance on the factors that a decision-maker acting on behalf of a patient should consider when deciding what is in a person's best interests.

Testamentary capacity

If you're asked to assess a patient's capacity to do anything other than make health and welfare decisions - for example, to write a will - please remember that this is a different test and should be approached with caution.

Read our guide to testamentary capacity for further advice. Members can contact us for help if they receive a request they're unsure about.

This page was correct at publication on 20/10/2022. Any guidance is intended as general guidance for members only. If you are a member and need specific advice relating to your own circumstances, please contact one of our advisers.

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