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Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Horizon Introduction There is a growing interest in the use of accelerometers and sensors embedded in implantable cardioverter-defibrillators (ICDs) for monitoring patient activity. Despite evidence regarding the potential clinical value of device-measured activity (D-PA), the validity of these measurements has not yet been established. Objective To assess the validity of device-measured activity against a research-grade, widely validated wearable accelerometer. Methods This is a subanalysis of the ongoing multicenter, prospective SafeHeart study. Raw accelerometry data was continuously sampled at 50Hz from a wrist-worn accelerometer (GENEActiv) during 12 months. Days with at least 22 hours of wear time were used to create summary measures of time in activity, daily active volume and total slow walking steps. These measures were compared to D-PA harmonised as percentage of active time per day, from four different ICD vendors’ remote transmission data, using linear mixed effect models. Results Wearable and device-measured activity data in 51 ICD patients rendered 1228 days (mean 24 days ±19) with both wearable and device-measured activity data. There were significant differences between wearable and device-measured accelerometery in the average time active per day (Table 1). For two vendors significant associations between D-PA, daily active volume, and total slow walking steps were observed. Also, associations between D-PA and daily active time and moderate vigorous physical activity were found in a third vendor. For the fourth vendor no association between any wearable activity metric and D-PA was found. Inter-patient differences accounted for 73.1% of the total variance in D-PA. Conclusion Results demonstrate substantial differences in device-measured activity measurements compared to research-grade activity data. This has implications for the utility and generalizability of D-PA as clinical parameter. Table 1. Summary of data