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

Authors

  • Agustín Escalante,

    Corresponding author
    1. The University of Texas Health Science Center at San Antonio, Texas
    • University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, Texas 78229-3900===

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  • Inmaculada Del Rincón,

    1. The University of Texas Health Science Center at San Antonio, Texas
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  • John E. Cornell

    1. The University of Texas Health Science Center at San Antonio and the South Texas Veterans Administration Health System, San Antonio, Texas
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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.

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.

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