Background About a half of patients with rheumatoid arthritis (RA) have normal or low C-reactive protein (CRP) levels1–3. Calprotectin is a promising and probably more specific biomarker of disease activity than conventionally used acute phase reactants.
Objectives The aim of this study was to analyse levels of serum calprotectin in RA patients with clinically active disease and with low CRP (<10 mg/L).
Methods A total of 160 RA patients and 32 healthy subjects were enrolled in this study. All patients underwent clinical examination (DAS28). The levels of calprotectin were analyzed in patients with moderate to high disease activity with low CRP levels and in healthy subjects. The discriminatory capacity of calprotectin to identify clinically active patients in spite of normal CRP was assessed using ROC curves.
Results Out of all RA patients, 74/160 (46.3%) had low CRP and were in remission or had low activity at the same time. However, 51/160 (32%) had low CRP levels despite moderate to high disease activity according to DAS28. In these patients, calprotectin levels were significantly higher than in patients with low CRP in remission or with low disease activity at the same time (mean 2.7±1.5 vs. 2.1±1.2 μg/mL, p=0.043) and differed from that in healthy subjects (mean 2.7±1.5 vs. ± 1.9±1.2 μg/mL, p=0.011) (Figure 1). The discriminatory capacity for calprotectin to distinguish clinically active vs. inactive patients in spite of low CRP using AUC of the DAS28 was 0.607 (95% CI 0.503 to 0.711, p=0.043) with an optimal cut-off 2.5 μg/mL.
Conclusions The present study demonstrates that calprotectin may reflect inflammatory activity in RA patients where CRP fails to do so.
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Acknowledgements This work was supported by a project of the Ministry of Health of the Czech Republic for conceptual research development by organization , Charles University Grant Agency (GAUK grant), Czech Republic  and Specific Academy Research Projects (SVV) [260 031].
Disclosure of Interest None declared