Can focusing on UPDRS Part II make assessments of Parkinson disease progression more efficient?

Harrison et al. have attempted to validate Part II of the Unified Parkinson's Disease Rating Scale (UPDRS II) as a medication-independent measure of disease progression. The authors collected cross-sectional data from a cohort of 888 patients with idiopathic Parkinson disease, and they found a robust association between UPDRS II scores and disease duration. Other variables considered were the patients' levodopa status, age at disease onset, and scores on UPDRS I, II and III. The results suggest that a single UPDRS II measurement might be a good indicator of progression at a given time point, irrespective of the current disease-related circumstances. This concept is attractive in its simplicity and patient-centeredness. However, this evidence came from a single-center, retrospective study, the statistical model was constructed using a nonvalidated surrogate as an independent variable, and no external replication was conducted. Until further confirmation, therefore, Harrison et al.'s proposal can only be considered to be a working hypothesis.