Background: In end-of-life situations, patients frequently lack the ability to make decisions. In France, the trusted person arrangement allows the elderly patients' voices to be heard even after the onset of cognitive decline. The testimony of the trusted person prevails over any other from family and friends. Objective: The aim of our study is to measure the propensity of informal caregivers of elderly patients to be formally designated as a trusted person, their propensity to act as a trusted person without formal designation, their understanding of this concept, as well as factors favouring trusted person behaviour and completion of designation formalities. Methods: We conduct an online survey of informal caregivers of elderly patients, based on a panel of caregivers of patients over 65 years old, residing in metropolitan France. Results: Most informal caregivers act as trusted persons. Formal appointment is not systematically made. Caregivers understand the core missions of trusted third parties, i.e. conveying the patient's medical wishes, accompanying them to consultations and providing advice. Nevertheless, confusion persists, with concepts such as contact person, caregiver and guardian. There is a lack of awareness of the procedures for appointing trusted persons. We find that the hospital is the most effective source of information. Finally, we show that the level of legal knowledge has no significant effect on the behaviors observed, while the intensity of the assistance provided is associated with a greater probability of assuming the role of trusted support person, all other things being equal. Conclusion: The lack of formalization is widespread in our sample, despite the fact that these missions are actually carried out in the field by the majority of caregivers. This raises the question of a possible reform of the system. The external validity of our results must be qualified, however, by the size of our sample