Objective. To compare the accuracy of serum calprotectin levels, CRP and ESR in stratifying disease activity in RA patients receiving tocilizumab (TCZ).
Methods. Cross-sectional study of 33 RA patients receiving TCZ. DAS28, Simplified Disease Activity Index, Clinical Disease Activity Index, joint counts and serum levels of CRP, ESR, calprotectin and TCZ were measured. Associations between calprotectin, ESR and CRP and articular indices were analysed by correlation and linear regression. The accuracy and discriminatory capacity of calprotectin was assessed by receiver operating characteristic curves (area under the curve).
Results. Calprotectin levels, but not CRP or ESR, were strongly correlated with all composite indices (all r coefficients over 0.50). Calprotectin, but not CRP or ESR, was significantly lower in patients in remission compared with those with low disease activity [1.57 μg/ml (s.d. 1) vs 3.35 μg/ml (s.d. 1), P = 0.001]. In a fully adjusted model (R2 = 0.82), DAS28-ESR increased 0.48 units per μg/ml calprotectin increase (P < 0.001). Using a DAS28 >3.2 as the reference variable, calprotectin showed an area under the curve of 0.922, and the best cut-off was 5.19 μg/ml (odds ratio 11.5). CRP levels, but not calprotectin, were dependent on detectable TCZ trough serum levels.
Conclusion. Calprotectin serum levels seem to be an accurate biomarker for assessing disease activity in RA patients receiving TCZ.