Study protocol for a factorial randomized trial of behavioral interventions to increase health care proxy documentation among older and seriously ill adults

Introduction Health care proxies serve as surrogate decision-makers for patients with limited decisional capacity, yet only one in three U.S. adults have designated one. This trial will evaluate low-cost, scalable behavioral interventions embedded in previsit mailed outreach to increase health care proxy documentation. Methods and analysis In this 2 x 2 + 1 factorial superiority trial, approximately 20,000 patients with an upcoming in-person primary care visit at a Northern California health system will be randomized over a 12-month period into one of four intervention mailers encouraging designation of a health care proxy, or to no mailer under usual care. Eligible patients will have no documentation of a health care proxy in the electronic health record (EHR) and will be aged 55 years or older, or 18 years or older with cancer, congestive heart failure, or chronic kidney disease. The four mailers differ by the presence or absence of a pre-commitment prompt, and whether the mailer is sent from the patient's primary care provider or the health system. The primary outcome is EHR-documentation of a health care proxy by 30 days after the index scheduled visit date. Secondary outcomes include proxy documentation through 90 days, legal advance care planning documentation through 30 and 90 days, and visit attendance. We will use logistic regression to estimate the intention-to-treat effect of receiving any intervention mailer versus no mailer, a pre-commitment mailer versus a mailer without pre-commitment, and a provider-signed versus health system-signed mailer. Ethics and dissemination The University of Southern California Institutional Review Board approved the protocol and granted a waiver of informed consent for this minimal-risk trial. Results will be posted on ClinicalTrials.gov within one year of the primary completion date. Trial Registration The trial was prospectively registered on December 12, 2025 at ClinicalTrials.gov (NCT07290478).