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Extended report
A stratified model for health outcomes in ankylosing spondylitis
  1. Pedro Machado1,2,
  2. Robert Landewé3,
  3. Jürgen Braun4,
  4. Kay-Geert A Hermann5,
  5. Xenofon Baraliakos4,
  6. Daniel Baker6,
  7. Ben Hsu6,
  8. Désirée van der Heijde2
  1. 1Department of Rheumatology, Coimbra University Hospital, Coimbra, Portugal
  2. 2Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands
  3. 3Department of Rheumatology, Academic Medical Center/Amsterdam University, Amsterdam and Atrium Medical Center, Heerlen, The Netherlands
  4. 4Department of Rheumatology, Rheumazentrum Ruhrgebiet Herne, Ruhr-University Bochum, Germany
  5. 5Department of Radiology, Charité Medical School, Berlin, Germany
  6. 6Research and Development, Centocor Inc, Malvern, Pennsylvania, USA
  1. Correspondence to Professor Désirée van der Heijde, Department of Rheumatology, Leiden University Medical Center, P O Box 9600, 2300 RC Leiden, The Netherlands; d.vanderheijde{at}


Objective To investigate the relationships between several health outcomes in ankylosing spondylitis (AS).

Methods Baseline pretreatment data from 214 patients with AS participating in the AS Study for the Evaluation of Recombinant Infliximab Therapy were analysed. Measures of health-related quality of life (HRQoL) and physical function were used as dependent variables in linear regression analysis. Associations between HRQoL (36-Item Short Form (SF-36)), physical function, clinical disease activity, spinal mobility, structural damage, MRI inflammation, disease duration, age, gender, body mass index and HLA-B27 were explored. Univariate associations were retested in multivariate models. The robustness of the models was evaluated by sensitivity analyses.

Results The physical component of SF-36 was independently associated with measures of physical function and disease activity (adjusted R2 (adjR2)=0.39–0.40). The mental component of SF-36 was independently associated with physical function (adjR2=0.07). Physical function was independently associated with measures of spinal mobility and disease activity (adjR2=0.39–0.45). Spinal mobility was hierarchically shown to be an intermediate variable between structural damage and physical function, while physical function was shown to be intermediate between spinal mobility and the physical component of SF-36.

Conclusion According to the proposed stratified model for health outcomes in AS, HRQoL is determined by physical function and disease activity, physical function is determined by spinal mobility and disease activity, and spinal mobility is determined by structural damage and inflammation of the spine. As more is learnt about how to measure AS, knowledge about the disease improves and better decisions can be made on the assessment and treatment of this disease.

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  • Funding PM was supported by the Fundação para a Ciência e a Tecnologia (FCT) grant SFRH/BD/62329/2009.

  • Competing interests None.

  • Ethics approval Ethics approval was obtained for the ASSERT trial, the database used in our analysis.

  • Provenance and peer review Not commissioned; externally peer reviewed.