Psychometric evaluation of the Mastocytosis Symptom Severity Daily Diary (MS2D2) in adults with nonadvanced systemic mastocytosis (NonAdvSM)

This study evaluated the measurement properties and ability to detect clinically meaningful change of the Mastocytosis Symptom Severity Daily Diary (MS2D2), a patient-reported outcome instrument designed to measure symptom severity in adults with nonadvanced systemic mastocytosis (NonAdvSM). Psychometric evaluation of the MS2D2 used blinded data from 230 adults with NonAdvSM enrolled in the Summit Phase 2 study (NCT05186753). The study comprised two parts: Part 1 (n = 54) for dose-finding and initial domain structure development, and Part 2 (n = 179) for efficacy evaluation, validation and longitudinal assessment. Analyses included item-level statistics, confirmatory factor analysis (CFA), internal consistency (Cronbach’s α), test–retest reliability (ICC), convergent and known-groups validity, responsiveness, and anchor-based estimation of meaningful change thresholds. Structural validity supported a four-domain, 11-item Total Symptom Score (TSS) framework. Internal consistency and test–retest reliability were excellent (α = 0.79–0.95; ICC = 0.95–0.98). Convergent validity was supported through strong correlations with disease-specific symptom and health-related quality of life (HRQoL) measures (r = 0.85 and r = 0.59). Known-groups validation revealed domain scores differing by 2–5 points between anchor severity strata and TSS by 2.8 points (all p < 0.05). Scores were sensitive to change, correlating (r = 0.39–0.74) with changes in patient status. TSS within-patient meaningful improvement thresholds were 1.6–2.2 points (0–10 scale) and 17.2–24.7 points (0–110 sum), with domains showing similar ranges. Percent change thresholds indicated meaningful improvement at approximately 30–40% reduction for both TSS and domain scores. MS2D2 is a reliable, valid, and responsive instrument for assessing symptom severity in NonAdvSM, with evidence supporting its use in clinical trials to evaluate treatment benefit. Nonadvanced systemic mastocytosis (NonAdvSM) is a rare condition where patients experience persistent symptoms that can greatly affect HRQoL Until now, there has been no reliable and valid method to measure symptom severity or track meaningful treatment changes. This study evaluated the Mastocytosis Symptom Severity Daily Diary (MS2D2), a patient-reported outcome tool designed for this purpose. We examined whether MS2D2 is reliable, valid, and sensitive to changes in symptoms over time. Using data from 230 adults with NonAdvSM, we confirmed that MS2D2 measures symptoms accurately, produces consistent results, and can detect clinically important improvements. We also established score ranges that indicate meaningful change for patients. These findings support MS2D2 as a scientifically sound instrument for use in clinical trials and practice. It can help researchers and clinicians assess treatment benefits and better understand how therapies impact patients’ daily lives.