Measuring Parkinson's disease over time: The real-world within-subject reliability of the MDS-UPDRS

Background: An important challenge in Parkinson's disease research is how to measure disease progression, ideally at the individual patient level. The MDS-UPDRS, a clinical assessment of motor and nonmotor impairments, is widely used in longitudinal studies. However, its ability to assess within-subject changes is not well known. The objective of this study was to estimate the reliability of the MDS-UPDRS when used to measure within-subject changes in disease progression under real-world conditions. Methods: Data were obtained from the Parkinson's Progression Markers Initiative cohort and included repeated MDS-UPDRS measurements from 423 de novo Parkinson's disease patients (median follow-up: 54 months). Subtotals were calculated for parts I, II, and III (in on and off states). In addition, factor scores were extracted from each part. A linear Gaussian state space model was used to differentiate variance introduced by long-lasting changes from variance introduced by measurement error and short-term fluctuations. Based on this, we determined the within-subject reliability of 1-year change scores. Results: Overall, the within-subject reliability ranged from 0.13 to 0.62. Of the subscales, parts II and III (OFF) demonstrated the highest within-subject reliability (both 0.50). Of the factor scores, the scores related to gait/posture (0.62), mobility (0.45), and rest tremor (0.43) showed the most consistent behavior. Conclusions: Our results highlight that MDS-UPDRS change scores contain a substantial amount of error variance, underscoring the need for more reliable instruments to forward our understanding of the heterogeneity in PD progression. Focusing on gait and rest tremor may be a promising approach for an early Parkinson's disease population.