title: Calculating clinical progression rates in Parkinson's disease: methods matter.
abstract: BACKGROUND
  Disease progression in Parkinson's disease is often calculated in data from cross-sectional studies, where a severity score (e.g. UPDRS-motor score) is divided by disease duration. While this intuitively may seem a plausible approach, it is uncertain if these rates are similar to those calculated from longitudinal data. The aim of this study is to examine if progression rates calculated according to both methods yield the same results.
  
  
  METHODS
  We calculated two progression rates in data from the PROPARK study: one where last follow-up SPES/SCOPA motor and activities-of-daily-living scores were divided by disease duration, and one in which baseline motor and activities-of-daily-living scores were subtracted from data collected at last follow-up, and where the difference was divided by the time that passed between both assessments. We subsequently calculated the rank order correlation between both approaches.
  
  
  RESULTS
  We found that progression rates calculated from cross-sectional data are 1.5-2 times higher than those calculated from longitudinal data, and that the correlation between both methods is <0.50.
  
  
  CONCLUSIONS
  Progression rates calculated from cross-sectional data not only overestimate actual progression, but also yield a different rank order. We also discuss potential explanations for the discrepancy between both methods and argue that the method of calculating progression rates in data from cross-sectional studies in PD should not be used.
authors.affiliation: Department of Neurology, Leiden University Medical Center, Leiden, The Netherlands. Electronic address: j.marinus@lumc.nl.
authors.forename: Johan
authors.initials: J
authors.lastname: Marinus
authors.affiliation: Department of Neurology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Jorine F
authors.initials: JF
authors.lastname: van der Heeden
authors.affiliation: Department of Neurology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Jacobus J
authors.initials: JJ
authors.lastname: van Hilten
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003430:Cross-Sectional Studies; D004185:Disability Evaluation; D018450:Disease Progression; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010300:Parkinson Disease / Q000503:physiopathology* / Q000628:therapy*; D012720:Severity of Illness Index; D016896:Treatment Outcome
keywords: Cohort studies; Methodology; Parkinson's disease; Progression