App-based symptom monitoring in dermatology: pooled prospective trials to quantify engagement, retention and ecological momentary assessment trajectories (2018–2025)

Background : Digital health monitoring delivered via smartphone applications and ecological momentary assessment (EMA) can capture high-frequency symptom trajectories in chronic inflammatory skin diseases, but real-world engagement and retention vary substantially across studies and can complicate inference. We pooled five prospective clinical trials to harmonize clinic visits, patient-reported outcomes (PROs), and smartphone app–derived EMA logs and to quantify engagement, retention, and clinical anchoring of EMA signals. Methods : We harmonized five prospective dermatology studies (psoriasis and chronic hand/foot eczema spanning from 2018 to 2025, including two randomized controlled trials and three non-randomized studies, conducted at University Hospital Würzburg and University Medical Center Mannheim, into a relational dataset with patient-, visit-, and EMA-level tables. All participants were provided access to the smartphone monitoring app; actual use was voluntary and quantified from timestamped logs. In the app-assigned cohort, baseline characteristics were summarized overall and by engagement group (no use vs any use). Early engagement was quantified as active days in the first 28 days after first activity and retention as time from first to last observed activity, summarized with Kaplan–Meier curves. EMA pruritus and pain trajectories were summarized weekly over 26 weeks and anchored to clinic symptom assessments at baseline and 6 months. Associations between early engagement and 6-month improvement were evaluated using adjusted models. Results : The harmonized baseline clinical dataset included 550 app-assigned participants (95.6% psoriasis). Baseline disease burden was moderate (median Dermatology Life Quality Index (DLQI) 6 [IQR (Interquartile range) 2–14]; median pruritus 2 [1–5]; median pain 1 [0–3]). Baseline characteristics were broadly comparable between engagement groups (no use n=211 vs any use n=339) with similar baseline PROs and symptoms. Engagement in the first 28 days was low and right-skewed (median 2 active days [1–4]), and retention showed early drop-off with heterogeneous retention across trials (retained 68.4% at day 28; 29.8% at day 182). Weekly EMA trajectories aligned with clinic anchors and separated by clinical response strata (ΔDLQI quintiles). Higher early engagement showed a graded association with greater adjusted 6-month improvement, with wider uncertainty at very high engagement. Conclusions: In a multi-trial pooled dermatology cohort, EMA symptom tracking was feasible and clinically interpretable when anchored to visit-based assessments, while engagement and retention varied substantially across studies. Harmonized, reproducible data structures integrating clinic-based assessments, PROs, and EMA logs can support effective trialing of digital interventions and enable robust quantification of engagement heterogeneity and between-visit symptom trajectories.