How is mental capacity assessed?

The Mental Capacity Act 2005 sets a specific test, and it is narrower than most families expect. A person lacks capacity only in relation to a particular decision at a particular time — someone may be unable to manage complex finances yet perfectly able to decide what to eat, where to go, or who visits.

The two-stage test

1. Diagnostic stage: is there an impairment of, or disturbance in, the functioning of the mind or brain? Dementia, brain injury, mental illness, learning disability and intoxication can all qualify.
2. Functional stage: as a result, is the person unable to understand the relevant information, retain it, use or weigh it in making the decision, or communicate the decision by any means?

Both stages must be satisfied before anyone can conclude that capacity is absent.

The principles that govern the assessment

• Presumption of capacity — never assumed absent because of age, disability or diagnosis.
• Support to decide — all practicable steps must be taken first: simpler language, visual aids, an interpreter, or simply waiting for a better time of day.
• The right to make unwise decisions — an eccentric or risky choice is not evidence of incapacity.
• Best interests for any decision made on the person’s behalf, and always the least restrictive option that achieves the purpose.

Who assesses, and what to do if nobody does

There is no single "capacity assessor" profession: the assessor is the person making the decision, whether a nurse arranging treatment, a social worker planning a placement or a carer supporting a daily choice. Complex, high-stakes or disputed cases may warrant a formal assessment by a psychiatrist or neuropsychologist.

If a professional assumes capacity without assessing, or refuses to assess before a significant decision, raise it as a formal complaint with the organisation and, if needed, with the relevant regulator — the GMC, NMC, HCPC or Social Work England.

Two boundaries are worth knowing. The Deprivation of Liberty Safeguards sit within this framework for people in care homes and hospitals whose care amounts to a deprivation of liberty. And the MCA is not the Mental Health Act 1983: the MCA governs decisions for people who cannot make them, whatever the cause, while the MHA governs compulsory treatment and detention for mental disorder. Where both could apply, the analysis needs specialist advice.