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Background: Compulsive hand washing is one of the most frequent compulsions and includes highly ritualized, repetitive hand motions. Developing an algorithm that can automatically detect compulsive washing with off-the-shelf wearable devices is a first step toward developing more sophisticated sensor-based assessments and micro-interventions that might ultimately supplement cognitive behavioral therapy for obsessive-compulsive disorder (OCD)., Objective: The main objective was to establish whether enacted compulsive hand washing can be distinguished from routine hand washing. This distinction will inform future research on the development of an algorithm that can automatically detect compulsive hand washing., Method: Twenty-one participants were trained individually to wash their hands according to 1 of 5 scripted hand-washing procedures that were based on descriptions of pathological compulsive washes and additionally to wash their hands as they usually would, while wearing a smartwatch. Washes were video recorded to obtain validation data. To generate a baseline model, we opted to extract well-known features only (mean and variance of each sensor axis). We tested four classification models: linear support vector machine (SVM), SVM with radial basis functions, random forest (RF), and naive Bayes (NB). Leave-one-subject-out cross-validation was applied to gather F1, specificity, and sensitivity scores., Results: The best-performing parameters were a classification window duration of 10 s, with a mean-variance feature set calculated from quaternions, rate of turn, and magnetic flux measurements. The detection performance varied with the particular enacted compulsive hand wash (F1 range: 0.65-0.87). Overall, enacted compulsive and routine hand washing could be distinguished with an F1 score of 79% (user independent), a sensitivity of 84%, and a specificity of 30%., Conclusions: Our analysis of the sensor data demonstrates that enacted compulsive hand washing could be distinguished from routine hand washing with acceptable sensitivity. However, specificity was low. This study is a starting point for a series of follow-ups, including the application in individuals diagnosed with OCD., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)