Feasibility of Repeated Patient-Reported Outcome Collection and Trial Design Implications for Structured Transition Care in Adolescents with Congenital Heart Disease: A Single-Center Pilot Randomized Controlled Study

Background/Objectives: Structured transition care models for adolescents with congenital heart disease (CHD) are increasingly advocated, but methodological evidence to support the design of adequately powered randomized trials remains limited. This pilot randomized study was designed primarily to assess the feasibility of repeated patient-reported outcome (PRO) collection and to generate empirical parameters for planning a future confirmatory trial, rather than to formally evaluate intervention efficacy. Methods: This was a single-center, parallel-group, pilot randomized controlled trial conducted at Meyer Children’s Hospital, Florence, Italy, within the TELEMACO project (NCT05713591). Twenty-three adolescents with CHD were randomized 1:1 to a structured transition care intervention (n = 11) or usual care (n = 12). PROs, including the SF-12 Physical (PCS12) and Mental (MCS12) Component Summaries, health engagement, life satisfaction, and healthcare needs, were collected at baseline and at 3, 6, 9, and 12 months. Pre-specified exploratory analyses addressed retention, missingness, linear mixed-effects models, intraclass correlation coefficients (ICCs), and sample size scenarios. Results: Retention at 12 months was 63.6% (intervention) and 58.3% (control), with substantially lower completion rates at intermediate assessments (T2–T3: 27–50%), directly affecting the reliability of longitudinal estimates at those time points. Mixed-effects models showed no significant time-by-group interaction for PCS12 (p = 0.13) or MCS12 (p = 0.39); unadjusted contrasts suggested nominally higher PCS12 values in the intervention group at selected assessments. ICCs were approximately 0 for PCS12 and 0.56 for MCS12, indicating fundamentally different variance structures. Conclusions: Repeated PRO collection was feasible, though retention across intermediate assessments was inconsistent. The pilot generated empirically grounded estimates for the design of a future confirmatory trial. Sample-size scenarios were highly sensitive to uncertainty in the PCS12 variability estimate, ranging from approximately 25 to 115 analyzable participants per group, depending on the true standard deviation. Within this pilot dataset, PCS12 at 12 months, analyzed cross-sectionally with baseline adjustment, emerged as a provisional endpoint option requiring further evaluation in an adequately powered confirmatory trial.