Measurement Properties of 2 Novel PROs, the Pompe Disease Symptom Scale and Pompe Disease Impact Scale, in the COMET Study

BACKGROUND AND OBJECTIVES: The Pompe Disease Symptom Scale (PDSS) and Impact Scale (PDIS) were created to measure the severity of symptoms and functional limitations experienced by patients with late-onset Pompe disease (LOPD). The objectives of this analysis were to establish a scoring algorithm and to examine the reliability, validity, and responsiveness of the measures using data from the COMET clinical trial. METHODS: The COMET trial was a randomized, double-blind study comparing the efficacy and safety of avalglucosidase alfa and alglucosidase alfa in patients with LOPD aged 16–78 years at baseline. Adult participants (18 years or older) completed the PDSS and PDIS daily for 14 days at baseline and for 2 weeks before quarterly clinic visits for 1 year after randomization using an electronic diary. Data were pooled across treatment groups for the current analyses. Factor analysis and inter-item correlations were used to derive a scoring algorithm. Test-retest and internal consistency analyses examined the reliability of the measures. Correlations with criterion measures were used to evaluate validity and sensitivity to change. Anchor and distribution-based analyses were conducted to estimate thresholds for meaningful change. RESULTS: Five multi-item domain scores were derived from the PDSS (Shortness of Breath, Overall Fatigue, Fatigue/Pain, Upper Extremity Weakness, Pain) and 2 from the PDIS (Mood, Difficulty Performing Activities). Internal consistency (Cronbach α > 0.90) and test-retest reliability (intraclass correlation >0.60) of the scores were supported. Cross-sectional and longitudinal correlations with the criterion measures generally supported the validity of the scores (r > 0.40). Within-patient meaningful change estimates ranging from 1.0 to 1.5 points were generated for the PDSS and PDIS domain scores. DISCUSSION: The PDSS and PDIS are reliable and valid measures of LOPD symptoms and functional impacts. The measures can be used to evaluate burden of LOPD and effects of treatments in clinical trials, observational research, and clinical practice. TRIAL REGISTRATION INFORMATION: ClinicalTrials.gov identifier: NCT02782741