Relative value of erythrocyte sedimentation rate and C-reactive protein inassessment of disease activity in ankylosing spondylitis

Our aim was to determine whether C-reactive protein (CRP) or erythrocyte se dimentation rate (ESR) is more appropriate in measuring disease activity in ankylosing spondylitis (AS). We studied 191 consecutive outpatients with A S in The Netherlands, France, and Belgium. Patients were attending secondar y and tertiary referral centers. The external criterion for disease activit y was: physician and patient assessment of disease activity on a visual ana log scale (VAS) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). In each measure we defined 3 levels of disease activity: no activ ity, ambiguous activity, and definite disease activity. The patients with A S (modified New York criteria) were divided into 2 groups: those with spina l involvement only (n = 149) and those who also had peripheral arthritis an d/or inflammatory bowel. disease (IBD) (n = 42). For each criterion of dise ase activity, the patients with no activity and with definite activity were included in receiver operator curves and used to determine cutoff values w ith the highest sensitivity and specificity. We also calculated Spearman co rrelations. The median CRP and ESR were 16 mg/l and 13 mm/h, respectively, in the spinal group and 25 mg/l. and 21 mm/h. respectively, in the peripher al/IBD group. In both groups the Spearman correlation coefficients between CRP and ESR were around 0.50, There was moderate to poor correlation betwee n CRF, ESR, and the 3 disease activity variables (0.06-0.48). Sensitivity f or both ESR and: CRP was 100% for physician assessment and between 44 and 7 8% for patient assessment of disease activity and the BASDAI, while specifi city was between 44 and 84% for all disease activity measures. The positive predictive values of CRP and ESR in our setting were low (0.15-0.69). We c onclude that neither CRP nor ESR is superior to assess disease activity.