The Patient-Determined Disease Steps scale is not interchangeable with the Expanded Disease Status Scale in mild to moderate multiple sclerosis

Background and purpose: The validity, reliability, and longitudinal performance of the Patient-Determined Disease Steps (PDDS) scale is unknown in people with multiple sclerosis (MS) with mild to moderate disability. We aimed to examine the psychometric properties and longitudinal performance of the PDDS. Methods: We included relapsing–remitting MS patients with an Expanded Disability Status Scale (EDSS) score of less than 4. Validity and test–retest reliability was examined. Longitudinal data were analysed with mixed-effect modelling and Cohen's kappa for concordance in confirmed disability progression (CDP). Results: We recruited a total of 1093 participants, of whom 904 had complete baseline data. The baseline correlation between PDDS and EDSS was weak (ρ = 0.45, p < 0.001). PDDS had stronger correlations with patient-reported outcomes (PROs). Conversely, EDSS had stronger correlations with age, disease duration, Kurtzke's functional systems and processing speed test. PDDS test–retest reliability was good to excellent (concordance correlation coefficient = 0.73–0.89). Longitudinally, PDDS was associated with EDSS, age and depression. A higher EDSS score was associated with greater PDSS progression. The magnitude of these associations was small. There was no concordance in CDP as assessed by PDDS and EDSS. Conclusion: The PDDS has greater correlation with other PROs but less correlation with other MS-related outcome measures compared to the EDSS. There was little correlation between PDDS and EDSS longitudinally. Our findings suggest that the PDDS scale is not interchangeable with the EDSS.