Abstract
Background Patient Determined Disease Steps(PDDS) is a common patient-reported outcome(PRO) in multiple sclerosis(MS). However, concerns remain over its validity and reliability in mild to moderate disability, longitudinal performance and confounding due to mood disorders.
Objectives We aimed to examine the psychometric properties of the PDDS in people with MS(pwMS), to explore longitudinal associations between expanded disability status scale(EDSS) and PDDS, and examine the impact of mood on PDDS.
Methods We prospectively enrolled relapsing-remitting multiple sclerosis(RRMS) patients with mild to moderate disability. Participants completed an iPad-based version of the PDDS, other PROs and EDSS 6-monthly. Test-retest reliability and validity was assessed. Longitudinal data was examined with mixed effect modelling.
Results We enrolled 904 RRMS patients with a median age of 40.9years, median EDSS of 1.5 and median PDDS of 0. The baseline correlation between PDDS and EDSS was weak. Correlations between functional systems(FS), age, disease duration and processing speed test(PST) and EDSS were stronger compared to PDDS. Conversely, correlations between PROs and PDDS were stronger. PDDS test-retest reliability was good to excellent. Longitudinally, PDDS was weakly associated with EDSS, depression and quality-of-life scores. Higher EDSS was associated with greater progression in PDSS. The magnitude of these associations were small.
Conclusion The PDDS differs from the EDSS in its psychometric properties, and should not be used interchangeably. The discordance in the aspects of disability that the PDDS measures may explain the small magnitude of longitudinal and cross-sectional associations. Modifying the PDDS to better detect under-reported symptoms such as bladder and bowel dysfunction may improve its validity.
