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Extended report
Misclassification of disease activity when assessing individual patients with early rheumatoid arthritis using disease activity indices that do not include joints of feet
  1. Marije F Bakker1,
  2. Johannes WG Jacobs1,
  3. Aike A Kruize1,
  4. Maaike J van der Veen2,
  5. Catharina van Booma-Frankfort3,
  6. Simone A Vreugdenhil4,
  7. Johannes WJ Bijlsma1,
  8. Floris PJG Lafeber1,
  9. Paco MJ Welsing1,5
  1. 1Department of Rheumatology and Clinical Immunology, University Medical Center Utrecht, Utrecht, The Netherlands
  2. 2Department of Rheumatology, St Jansdal Hospital, Harderwijk, The Netherlands
  3. 3Department of Rheumatology, Diakonessenhuis, Utrecht, The Netherlands
  4. 4Department of Rheumatology, St Antonius Hospital, Nieuwegein, The Netherlands
  5. 5Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands
  1. Correspondence to Marije F Bakker, University Medical Center Utrecht, Department of Rheumatology and Clinical Immunology, F02.127, PO Box 85500, 3508 GA Utrecht, The Netherlands; m.f.bakker-8{at}


Objective To study whether assessment of rheumatoid arthritis (RA) disease activity in individual patients using the disease activity score in 28 joints (DAS28) or other instruments excluding joints of feet may lead to misclassification of disease activity.

Methods A cohort of RA patients was classified into three ‘regional radiographic damage progression’ groups: predominantly progression in feet, similar progression in hands and feet and predominantly progression in hands; both in early (0–2 years) and later (2–5 years) disease. Baseline and mean DAS28, individual DAS28 variables and tender joint counts (TJC) and swollen joint counts (SJC) of the feet were compared between groups. The longitudinal relation of DAS28 with radiographic damage was investigated using a mixed model analysis with rheumatoid factor status, baseline joint damage and TJC and SJC of the feet as covariates.

Results Early (n=265) and later (n=200) in the disease course, by definition, the classification procedure resulted in 25% as predominantly foot, 25% as predominantly hand and 50% as similar progressors. In early RA predominantly foot progressors had higher TJC and SJC of the feet compared with predominantly hand progressors (p<0.001), but DAS28 was similar. This was not seen in later disease. The longitudinal relation between DAS28 and radiographic progression was influenced by the region of progression (predominantly foot progressors vs others, p<0.001), suggesting that DAS28 underestimates disease activity in predominantly foot progressors. In this group, joint counts for the feet were independently related to radiographic progression.

Conclusions DAS28 underestimates actual disease activity and expected joint damage of individual early RA patients predominantly with disease in the feet.

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  • Competing interests None.