Latent variable approach to the measurement of physical disability in rheumatoid arthritis

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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. Conclusion. 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.

Original languageEnglish (US)
Pages (from-to)399-407
Number of pages9
JournalArthritis Care and Research
Issue number3
StatePublished - Jun 15 2004


  • Disease-specific health measures
  • Factor analysis
  • Generic health measures
  • Outcome assessment
  • Physical disability
  • Rheumatoid arthritis

ASJC Scopus subject areas

  • Rheumatology


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