Treatment first - emergency treatment and clinical care should be the first priority.
Consent / capacity – for each relevant decision about medical treatment, an assessment of the patient’s capacity (per the MCA) will determine whether it is something to which they can consent / refuse, or whether it is a best interests decision. Even lifesaving treatment will be unlawful if it is given to a patient who has capacity to consent and refuses it (or where there is a valid and effective Advance Decision to Refuse Treatment that has the same effect).
Best interests decision making – where a patient lacks capacity for a particular medical treatment decision (and there is no effective ADRT to pre-determine this) then a best interests decision must be made. The lead clinician responsible for the treatment is likely to be the decision maker (unless there is a relevant Lasting Power of Attorney or deputy), but in any case decision making should be collaborative, and aim to determine the patient’s best interests in the round (including social and emotional, as well as purely medical aspects). It should be informed by the patient’s own wishes, feelings, beliefs and values, and consultation as much as practicable with those engaged in caring for the patient or interested in their welfare, especially to enhance an understanding of the patient as an individual. [Note that there is no particular special role or status in this for those designated in lay terms as “next of kin”]. In some cases an Independent Mental Capacity Advocate (IMCA) can assist.