title: Reference curves of radiographic damage in patients with rheumatoid arthritis: application of quantile regression and fractional polynomials.
abstract: OBJECTIVE
  To (1) introduce the methodology of quantile regression and fractional polynomials; (2) test the application of this methodology to develop, conditional on disease duration, preliminary reference curves of radiographic damage in patients with rheumatoid arthritis (RA); and (3) prove the importance of the definition and selection of the reference group when developing reference curves.
  
  
  METHODS
  The study design was cross sectional. The main study factors were disease duration and radiographic damage using the Larsen score. The 2 study samples were 98 patients from a multicenter trial of cyclosporine and 203 patients with RA from a teaching hospital clinic.
  
  
  RESULTS
  Using disease duration as the time dependent covariate we constructed quantile regression reference curves of radiographic damage. The reference curves for the 2 samples differed in shape, location, and slope.
  
  
  CONCLUSIONS
  Quantile regression and fractional polynomials simplify the construction of reference curves when data cannot be easily modified to meet assumptions of normality, linearity, and constant variance. Quantile reference curves provide clinicians with a useful clinical tool to measure outcome at arbitrary timepoints, to interpret change, and to set treatment objectives. However, the definition and selection of the reference used to construct the reference curves is of critical importance.
authors.affiliation: Department of Rheumatology, The St. George Hospital, University of New South Wales, Sydney, Australia.
authors.forename: M
authors.initials: M
authors.lastname: Lassere
authors.forename: D
authors.initials: D
authors.lastname: Houssein
authors.forename: D
authors.initials: D
authors.lastname: Scott
authors.forename: J
authors.initials: J
authors.lastname: Edmonds
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001172:Arthritis, Rheumatoid / Q000000981:diagnostic imaging*; D001175:Arthrography / Q000379:methods / Q000592:standards*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010044:Outpatient Clinics, Hospital; D012015:Reference Standards; D012044:Regression Analysis