Movement Disorder Society Unified Parkinson's Disease Rating Scale Part III cannot Be Reliably Reduced for Predicting Time to Initiation of Parkinsonian Medication in Early Parkinson's Disease: An Item‐Level Sensitivity Analysis

BACKGROUND: The sum of clinician-rated MDS-UPDRS Part III Motor Examination items is widely used in Parkinson's disease (PD), but item- or subset prognostic value remains unclear. Whereas reduced subsets of patient-reported Parts IB + II show improved utility for predicting time to PD medication initiation, the validity of similar Part III subsets is unknown. OBJECTIVES: Determine whether a subset of MDS-UPDRS Part III items is superior in prognostic value to the full scale for time to PD medication initiation in early disease. METHODS: Harmonized longitudinal data from six early PD cohorts, with time to PD medication initiation as the primary outcome, were analyzed using a longitudinal item response theory model to rank items, combining discrimination and information. Rank-driven cumulative subsets were formed to analyze the primary outcome using time-varying Cox models under clinically relevant follow-up windows. Subset performance was assessed using the C-index and compared with the full-item model using Wald tests. Successful item reduction required stable, significant improvement across all follow-up windows. RESULTS: No item subset met the predefined criteria for stable and superior prognostic performance relative to the full Part III scale. Best reduced-subset versus full-model C-indices were similar and non-significant: 0.582 versus 0.581 under full follow-up, 0.602 versus 0.602 at 1 year, and 0.586 versus 0.585 at 2 years. CONCLUSIONS: Unlike Parts IB and II, no sensitivity-driven reduced subset significantly and consistently outperformed the full MDS-UPDRS Part III in predicting time to PD medication initiation in early PD, not supporting a reduction of the 33-item scale in this setting.