Johan Rönnelid

Objectives The aim of the present study was to evaluate the performance of serum and SF levels of the granulocyte protein calprotectin as an inflammatory biomarker in RA patients with knee synovitis. Methods Seventy-six RA patients with ongoing knee synovitis were included. Data on DAS with 28 joints and their subcomponents and radiological destruction of the affected knee were collected. White blood cell count, CRP, ACPA against cyclic citrullinated peptide version 2 (anti-CCP2), IgM RF and calprotectin were analysed in parallel in circulation and in SF. Counts of polynuclear and mononuclear cells were measured in SF. Results Serum (S)-calprotectin correlated more strongly than SF-calprotectin with inflammatory markers and disease activity. Instead, SF-calprotectin showed a strong correlation to SF counts of white blood cells, and especially to polymorphonuclear cell counts (Spearman’s ρ = 0.72, P < 0.001). S-calprotectin showed markedly stronger correlation with inflammatory markers and disease activity in ACPA positive as compared with ACPA negative RA patients; a similar difference was observed for patients with and without IgM RF. Conclusion The particularly strong association between circulating calprotectin and inflammation in ACPA positive RA is a new argument for a specific role for polymorphonuclear granulocytes/neutrophils in this RA subset. Measurement of calprotectin in SF does not convey any additional benefit compared with measurement in the circulation in RA patients with knee synovitis.