title: Latent variable approach to the measurement of physical disability in rheumatoid arthritis.
abstract: OBJECTIVE
  To measure physical disability in rheumatoid arthritis (RA) using a latent variable derived from a generic and a disease-specific self-reported disability instrument and an observer-assessed functional status scale.
  
  
  METHODS
  Consecutive patients with RA completed the modified Health Assessment Questionnaire (M-HAQ) and the Short Form 36 (SF-36) physical function scale. An observer assigned a Steinbrocker functional classification. We used principal component factor analysis to extract a latent variable from the 3 scales. We used the Bayesian Information Criterion to compare how well the new latent variable and the 3 primary scales fit the criterion standards of current work status; vital status at 6 years; grip strength; walking velocity; the timed-button test; pain; and joint tenderness, swelling, and deformity.
  
  
  RESULTS
  Complete data were available for 776 RA patients. The extracted latent variable explained 75% of the variance in the 3 primary scales. On a scale of 0-100, higher scores representing less disability, its mean +/- SD was 56.4 +/- 22.5. Correlation between the latent variable and the M-HAQ was -0.87; between the latent variable and SF-36 physical function scale was 0.89, and between the latent variable and Steinbrocker class was -0.85. Multivariate models that included the latent variable had superior fit than did models containing the primary scales for the criteria of current working; death by 6 years; pain; joint tenderness, swelling, or deformity; grip strength; walking velocity; and timed button test.
  
  
  CONCLUSIONS
  A latent variable derived from the M-HAQ, the SF-36 physical function scale, and the Steinbrocker functional class provides a parsimonious scale to measure physical disability in RA. The fit of the latent variable to comparison standards is equivalent or superior to that of the primary scales.
authors.affiliation: The University of Texas Health Science Center at San Antonio, 78229-3900, USA.
authors.forename: Agustín
authors.initials: A
authors.lastname: Escalante
authors.forename: Inmaculada
authors.initials: I
authors.lastname: Del Rincón
authors.forename: John E
authors.initials: JE
authors.lastname: Cornell
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001172:Arthritis, Rheumatoid / Q000503:physiopathology*; D004185:Disability Evaluation; D005163:Factor Analysis, Statistical; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D011795:Surveys and Questionnaires