OBJECTIVES: To psychometrically evaluate the Indolent Systemic Mastocytosis Symptom Assessment Form (ISM‑SAF©) scores among patients with ISM and smoldering systemic mastocytosis (SSM).

METHODS: An observational study was conducted in the US among eligible participants self-reporting an ISM or SSM diagnosis (a subset provided medical records to clinically confirm diagnosis), using an online platform. The ISM-SAF consists of 12 items and was completed daily throughout the study (Days 1–15). Other assessments were the Functional Assessment of Cancer Therapy–Cognitive Function (FACT-Cog) (Day 1), Mastocytosis Quality of Life (MC‑QoL), Short Form Health Survey (SF‑12v2®), and Patient Global Impression of Severity (PGIS) (each completed on Days 1 and 15).

RESULTS: 103 subjects contributed to analyses (81.6% female; mean age=50.2 [±12.6]) with no obvious differences between those with self-reported (n=45) and confirmed (n=58) diagnoses. The ISM-SAF was evaluated in terms of item scores, a Total Symptom Score (TSS), a Gastrointestinal Symptom Score (GSS), and a Dermal Symptom Score (DSS). Subjects reported symptom severity across the range of responses. Internal consistency reliability (α) for the TSS, GSS, and DSS biweekly scores was 0.84, 0.78, and 0.67, respectively. Test-retest reliability among patients reporting no symptom change from Day 1 to 15 was assessed using the intra-class correlation coefficient (and 95% confidence interval) comparing Week 1 to Week 2 TSS, GSS, and DSS; results were 0.96 (0.94–0.98), 0.94 (0.89–0.96), and 0.96 (0.94–0.98), respectively. Construct validity and known-groups analysis showed ISM‑SAF scores as strongly correlated to variables assessing symptoms and physical function, less strongly to variables assessing distal disease impacts, and able to distinguish among clinically unique groups specified by the PGIS (p<0.001), SF‑12v2 (p<0.001), and MC‑QoL (p<0.001).

CONCLUSIONS: The ISM-SAF is a content-valid assessment of symptoms associated with ISM and SSM that can produce reliable and construct-valid scores in its target patient population.