The design of the intervention is an ongoing process which will consist of two phases: a determination of the intervention using the Delphi-modified consensus method and an assessment of its acceptability, as described in the methodological recommendations of the Medical Research Council Guidance.

First phase : determination of the intervention.

A list of proposed interventions drawn from the literature and the exploratory work will be proposed to panels of participants using the DELPHI-modified method : Rounds made up of a panel of carers, a panel of healthcare professionals in the field, and a panel of academic experts who have knowledge of the scientific literature on the subject as well as having clinical experience.

The method consists of proposing different intervention proposals and having them validated/modified using a Likert scale by scoring during different e-Delphi rounds (a paper version of the questionnaire will also be available). Following each Delphi round, a meeting will be organised with all the members of each panel to discuss the comments made during these rounds.

• Each item will be graded from 1 to 9 by each participant.
• Topics with a 70% score ≥ 7 are retained.
• Topics with a score of 70% ≤ 3 are eliminated.
• Topics scoring 70% between 4 and 6 are reformulated by the members of the scientific committee and reproposed in the following round.

It usually takes between 3-4 rounds to eliminate/validate themes.

Second phase : assessment of the acceptability of the conceived intervention.

Following this consensus, a complex intervention with several components will be determined and proposed by the scientific committee to the 3 participating CPTS (territorial professional health communities): the Asclepios and Est Cher CPTS (which had already participated in the previous exploratory studies) and the Iroise CPTS (naïve to any preliminary assessment of its needs in its territory). Quantitative data on the acceptability of the intervention will then be collected, namely :

• the number of carers who were offered or not offered the intervention
• the number of carers who benefited from the intervention in its entirety or by component
• the number of carers who refused or abandoned the intervention. Qualitative data on acceptability will also be collected through observations of voluntary carers and healthcare professionals and through interviews. Throughout the process of creating the intervention (from the Delphi rounds to the proposal of the intervention to the 3 participating CPTS), an exploration of the co-construction process and the acceptability of the intervention will be carried out using qualitative Focus Group (FG) interviews with each group involved (carers, healthcare professionals, university experts), then with each CPTS (carers and healthcare professionals from each CPTS). Depending on the results of this exploration, the intervention may be adapted by the scientific committee. This is a continuous process of evaluating the intervention, as recommended in the development of complex interventions.

The whole complex intervention thus determined will be reported using the TIDieR checklist model

Inclusion Criteria:

• Healthcare professionals who are members of the 3 CPTSs included and who agree to participate
• Adult informal carers of patients living with Alzheimer's disease or a related dementia from the 3 CPTS who agree to participate
• University experts: general practitioners, geriatric mobile team doctor, advanced practice nurse, clinical psychologist, biostatistical methodologist. Some of them had already taken part in the steering committee for ISAMA's exploratory qualitative and quantitative studies.

Exclusion Criteria:

- Professional carers

Determining the intervention using the DELPHI-modified method, the 30 participants will be academic experts (10 people) and people from the 2 professional communities (CPTS) that took part in the exploratory studies, Asclepios and Est Cher (10 primary healthcare professionals and 10 carers). The 10 carers will have different characteristics depending on the level of dependency and the extent of the behavioural problems of the patient they are helping, and on their family status (parent, child or other). They will also come from the care areas of the 2 CPTS, Asclepios and Est Cher.

Concerning the qualitative sociological exploration of the process of acceptability and co-construction of the intervention, the 30 participants will be academic experts (the 10 people identified above) and people from the 3 CPTS, Asclepios, Est Cher and Iroise who tested the conceived intervention (10 healthcare professionals and 10 carers).