Calculating the ankylosing spondylitis disease activity score if the conventional c-reactive protein level is below the limit of detection or if high-sensitivity c-reactive protein is used: an analysis in the DESIR cohort

Arthritis Rheumatol. 2015 Feb;67(2):408-13. doi: 10.1002/art.38921.


Objective: The Ankylosing Spondylitis Disease Activity Score (ASDAS) is a composite measure of disease activity in axial spondyloarthritis. The aims of this study were to determine the most appropriate method for calculating the ASDAS using the C-reactive protein (CRP) level when the conventional CRP level was below the limit of detection, to determine how low CRP values obtained by high-sensitivity CRP (hsCRP) measurement influence ASDAS-CRP results, and to test agreement between different ASDAS formulae.

Methods: Patients with axial spondyloarthritis who had a conventional CRP level below the limit of detection (5 mg/liter) were selected (n = 257). The ASDAS–conventional CRP with 11 different imputations for the conventional CRP value (range 0–5 mg/liter, at 0.5-mg/liter intervals) was calculated. The ASDAS-hsCRP and ASDAS using the erythrocyte sedimentation rate (ESR) were also calculated. Agreement between the ASDAS formulae was tested.

Results: The ASDAS-hsCRP showed better agreement with the ASDAS-CRP calculated using the conventional CRP imputation values of 1.5 and 2.0 mg/liter and with the ASDAS-ESR than with other imputed formulae. Disagreement occurred mainly in lower disease activity states (inactive/moderate disease activity). When the CRP value was <2 mg/liter, the resulting ASDAS-CRP scores may have been inappropriately low.

Conclusion: When the conventional CRP level is below the limit of detection or when the hsCRP level is <2 mg/liter, the constant value of 2 mg/liter should be used to calculate the ASDAS-CRP score. There is good agreement between the ASDAS-hsCRP and ASDAS-ESR; however, formulae are not interchangeable.

Publication types

  • Comparative Study
  • Multicenter Study

MeSH terms

  • Biomarkers / blood
  • Blood Sedimentation
  • C-Reactive Protein / metabolism*
  • Cohort Studies
  • France
  • Humans
  • Limit of Detection*
  • Mathematics
  • Models, Biological*
  • Sensitivity and Specificity
  • Severity of Illness Index*
  • Spondylitis, Ankylosing / blood*
  • Spondylitis, Ankylosing / diagnosis*


  • Biomarkers
  • C-Reactive Protein