[A study on calculating the ankylosing spondylitis disease activity score according to the C-reactive protein level]

Abstract

Objective: To study the prediction value of C-reactive protein (CRP) level to ankylosing spondylitis disease activity score (ASDAS), the most widely used activity score in ankylosing spondylitis (AS).

Methods: A total of 386 patients with AS were enrolled and divided into 2 groups by CRP ≥ 3.5 mg/L (n=266) and CRP<3.5 mg/L(n=120). ASDAS-CRP was evaluated in patients with CRP below the normal range (3.5 mg/L) in different subgroups. Three methods were used to test the consistency between ASDAS-CRP and ASDAS-erythrocyte sedimentation rate (ESR).

Results: The ASDAS-CRP showed a good correlation with the ASDAS-ESR when CRP level was ≥ 3.5 mg/L (r=0.899, P=0.000). In the group of CRP <3.5 mg/L, ASDAS-ESR showed better consistency with ASDAS-CRP when CRP level was below 1.5 mg/L(intra-class correlation coefficient 0.902; kappa coefficient 0.70). By the evaluation of variant gradient matrix, CRP 1.5 mg/L coincided with disease activity states.

Conclusion: There is good consistency between ASDAS-CRP and ASDAS-ESR. When CRP level is below the normal range, 1.5 mg/L could be a cut-off value to calculate the optimal ASDAS-CRP score.