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STUDY DESIGN Prospective cohort study. OBJECTIVE To compare feasibility of self-reported and proxy-reported youth version of EuroQoL Five-Dimension Three-Level Questionnaire (EQ-5D-3L-Y), to estimate the agreement of health outcome between patients with adolescent idiopathic scoliosis (AIS) and their proxies, and to examine factors that may affect patient-proxy agreement. SUMMARY OF BACKGROUND DATA The EQ-5D-3L-Y questionnaire has both self-reported and proxy-reported versions. Despite previous studies have indicated that proxies tended to respond with higher or lower levels of severity in specific dimensions than patients report, the level of agreement between children with AIS and their proxies remained unknown. METHODS A consecutive sample of patients with AIS and their caregivers were recruited. Feasibility was tested according to the proportion of missing responses. Agreements between self-report and proxy EQ-5D-3L-Y were evaluated using percentage agreement, Gwet agreement coefficients and the intraclass correlation coefficients. Linear regressions and logistic regressions were conducted to assess the factors associated with the agreement in health outcome between self-reported and proxy-reported EQ-5D-3L-Y. RESULTS A total of 130 patient-proxy pairs were involved in the study. Agreement of EQ-5D-3L-Y responses between the self-report and proxy version was good for "Feeling worried/sad/unhappy" dimension, and very good for other dimensions. Poor agreement in visual analog scale score was observed between patient and proxy versions. Proxy's education level, patient's curvature type, and treatment modality were the significant determinants of the agreement in "Mobility," "usual activities," and "pain/discomfort" dimension, respectively. CONCLUSION Proxy-reported EQ-5D-3L-Y demonstrates good feasibility and satisfactory agreement with patient version. Proxy's education appears to have positive influence in agreement between patient-proxy dyads. LEVEL OF EVIDENCE 2.