title: 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.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.forename: Pedro
authors.initials: P
authors.lastname: Machado
authors.forename: Victoria
authors.initials: V
authors.lastname: Navarro-Compán
authors.forename: Robert
authors.initials: R
authors.lastname: Landewé
authors.forename: Floris A
authors.initials: FA
authors.lastname: van Gaalen
authors.forename: Christian
authors.initials: C
authors.lastname: Roux
authors.forename: Désirée
authors.initials: D
authors.lastname: van der Heijde
mesh_terms: D015415:Biomarkers / Q000097:blood; D001799:Blood Sedimentation; D002097:C-Reactive Protein / Q000378:metabolism*; D015331:Cohort Studies; D005602:France; D006801:Humans; D057230:Limit of Detection; D008433:Mathematics; D008954:Models, Biological; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D013167:Spondylitis, Ankylosing / Q000097:blood* / Q000175:diagnosis*
chemical_list: D015415:Biomarkers; D002097:C-Reactive Protein