How to calculate the ASDAS if the conventional CRP is below the limit of detection or if using high sensitivity CRP? – An analysis in the DESIR cohort

Abstract

Background: The Ankylosing Spondylitis Disease Activity Score (ASDAS) is a composite measure of disease activity in axial spondyloarthritis.

Objectives: Our aims were to investigate the most appropriate ASDAS-C-reactive protein (ASDAS-CRP) calculation method when the conventional CRP (cCRP) is below the limit of detection, to study the arithmetic influence of low CRP values obtained by high sensitivity CRP (hsCRP) in ASDAS-CRP results and to test agreement between different ASDAS formulae.

Methods: Patients with axial spondyloarthritis and cCRP below the limit of detection (5mg/L, n=257) were selected. ASDAS-cCRP was calculated using eleven imputation strategies for the cCRP (range 0-5, at 0.5 intervals). ASDAS-hsCRP and ASDAS-ESR were also calculated. Agreement between ASDAS formulae was tested.

Results: ASDAS-CRP calculated with the cCRP imputation values of 1.5 and 2.0mg/L and ASDAS-erythrocyte sedimentation rate (ESR) had better agreement with ASDAS-hsCRP than other imputed formulae. Disagreement was mainly in lower disease activity states (inactive/moderate disease activity). When the CRP value is <2mg/L, the CRP component of the ASDAS-CRP formula can take very low values that may result in inappropriately low ASDAS-CRP values.

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