["Health Services Research, Volume 61, Issue 3, June 2026. ", "\nABSTRACT\n\nObjective\nTo examine how patient engagement with SMS text reminder scan mediate cancer screening completion among patients at Federally Qualified Health Center clinics (FQHCs).\n\n\nStudy Setting and Design\nPatients overdue for cancer screening were randomly assigned to one of two groups: a 3‐week interactive SMS intervention or a 6‐week theory‐informed SMS intervention. The binary outcome was cancer screening completion after 90 days of SMS intervention. A mediation model was used to assess associations between text‐message response rates and cancer screening completion. Exposure variables included sociodemographic characteristics, type of SMS intervention, type of cancer screening needed, and health status variables.\n\n\nData Sources and Analytic Sample\nData were obtained from two large FQHC networks in Texas and California in 2023. We included 4,344 patients who participated in the 3‐week or 6‐week intervention groups, successfully received all interactive messages, had unique phone numbers, and did not opt out of the intervention.\n\n\nPrincipal Findings\nOverall effects were statistically significant, indicating partial mediation. Full mediation was observed for insurance type: compared with Medicaid, private insurance was associated with a 5‐percentage‐point (PP) higher predicted probability of screening completion (Average marginal effect [AME] = 5.0 PP, 95% CI [0.01, 0.09]). Partial mediation was observed for Medicare (AME = 7.0 PP, 95% CI [0.02, 0.13]), uninsurance (AME = 5.0 PP, 95% CI [0.01, 0.09]), patient with diabetes (AME = −4.0 PP, 95% CI [−0.08, −0.01]), non‐primary English language (AME = 8.0 PP, 95% CI [0.05, 0.10], p < 0.001), and large‐city residence (AME = 9.0 PP, 95% CI [0.07, 0.10]), all of which were associated with higher predicted probabilities of screening completion.\n\n\nConclusions\nEngagement with SMS reminders was a significant mediator of cancer screening completion, suggesting that increasing message interactivity may improve cancer screening uptake. Theory‐informed interactive messaging may promote greater patient engagement.\n\n"]