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ObjectivesDeveloping disease modifying therapies for Parkinson's disease (PD) calls for outcome measurement strategies focused on characterizing early stage disease progression. We explored the psychometric evidence for using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) part II (patient motor experience of daily living) and part III (clinician motor examination) in this context.MethodsMDS-UPDRS-II and -III data were collected at screening, month 12, and month 24 from 384 early stage PD patients (diagnosis ≤ 2 years; Hoehn and Yahr stage 1/2) in the Parkinson's Progression Markers Initiative (PPMI) study. Psychometric analysis, based on Rasch measurement theory (RMT), was performed on both the original MDS UPDRS-II and -III scales and exploratory content-driven scale structures.ResultsRMT analyses showed neither scale was well targeted to early PD. A marked floor effect appeared for most items and a clear item gap was consistently observed in very mild severity of motor signs and levels of motor impact. The original MDS-UPDRS-II and -III scales also displayed disordered thresholds (9/13 and 20/33 items, respectively), indicating response scales not functioning as expected, and misfit (5/13 and 12/33 items, respectively), flagging areas for potential improvement.ConclusionsThe MDS-UPDRS-II and -III have psychometric limitations which limits the precision of measurement of motor symptoms and impact in early PD. This can lead to insensitivity in detecting differences and clinical change. Importantly, the diagnostic psychometric evidence provided by the RMT analysis provides a clear starting point for how to improve the quantification of clinically relevant concepts to characterize the course of early PD.
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