BackgroundPatients experiencing systemic patterns of disadvantage, such as racial/ethnic minorities and those with limited English proficiency, are underrepresented in research. This is particularly true for large pragmatic trials of potentially sensitive research topics, such as advance care planning (ACP). It is unclear how phone outreach may affect research participation by underrepresented individuals.ObjectiveTo assess the effect of phone outreach, in addition to standard mail survey recruitment, in a population-based ACP pragmatic trial at three academic health systems in California.DesignRetrospective cohort studyPatientsPrimary care patients with serious illness were mailed a survey in their preferred language. Patients who did not initially respond by mail received up to three reminder phone calls with the option of survey completion by phone.Main measuresEffect of phone outreach on survey response rate associated with respondent demographic characteristics (e.g., Social Vulnerability Index [SVI], range 0 (low) to 1 (high)).ResultsAcross the health systems, 5998 seriously ill patients were mailed surveys. We obtained completed surveys from 1215 patients (20% response rate); 787 (65%) responded after mail alone and 428 (35%) participated only after phone outreach. Patients recruited after phone outreach compared to mail alone were more socially vulnerable (SVI 0.41 v 0.35,P< 0.001), were more likely to report being a racial/ethnic minority (35% v 28%,P= 0.006), and non-English speaking (16% v 10%,P= 0.005). Age and gender did not differ significantly. The inclusion of phone outreach resulted in a sample that better represented the baseline population than mail alone in racial/ethnic minority (28% mail alone, 30% including phone outreach, 36% baseline population), non-English language preference (10%, 12%, 15%, respectively), and SVI (0.35, 0.37, 0.38, respectively).ConclusionsPhone outreach for a population-based survey in a pragmatic trial concerning a potentially sensitive topic significantly enhanced recruitment of underrepresented seriously ill patients.