title: The Health Assessment Questionnaire Disability Index is a valid measure of physical function in gout.
abstract: OBJECTIVE
  There are no disability instruments that have specifically been validated for gout. The aim of this study was to determine the construct validity of the Health Assessment Questionnaire Disability Index (HAQ-DI) in gout and the internal validity using Rasch analysis.
  
  
  METHODS
  An observational cohort study of two groups of clinic patients with gout (n=20, n=53), in which clinical and functional measures were correlated with HAQ-DI scores. Rasch analysis was used to determine the internal validity of summated scores as a measure of physical disability.
  
  
  RESULTS
  The HAQ-DI items fitted a Rasch measurement model, confirming internal validity of the scale, although there was evidence of disordered thresholds and rescoring items as a 3-option response rather than a 4-option response improved model fit and resolved the disordered thresholds. HAQ-DI scores showed a bimodal distribution and evidence of floor effects. Clinical indices correlated highly with HAQ-DI scores in gout patients, particularly other measures of physical function. A strong relationship between days of sick leave and HAQ-DI was observed in gout patients (r2=0.44, p<0.001).
  
  
  CONCLUSIONS
  It is concluded that HAQ-DI has good construct and internal validity in gout but a modified scoring approach better fits a Rasch model.
authors.affiliation: Department of Medicine, University of Otago, Wellington, New Zealand. will.taylor@otago.ac.nz
authors.forename: W J
authors.initials: WJ
authors.lastname: Taylor
authors.forename: K
authors.initials: K
authors.lastname: Colvine
authors.forename: K
authors.initials: K
authors.lastname: Gregory
authors.forename: J
authors.initials: J
authors.lastname: Collis
authors.forename: F M
authors.initials: FM
authors.lastname: McQueen
authors.forename: N
authors.initials: N
authors.lastname: Dalbeth
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D004185:Disability Evaluation; D005260:Female; D006073:Gout / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics