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Introduction: Calprotectin (CLP, S100A8A9) is secreted by neutrophils and monocytes during systemic inflammation due to activation of innate immunity. We aim to evaluate serum CLP (sCLP) levels in patients with adult onset Still’s disease (AOSD) and idiopathic recurrent pleuropericarditis (PP) in active and inactive disease.

Methods: This is a monocentric, retrospective trial with 130 adult patients (PP=60; AOSD=70) recruited between January 2021 and January 2024. The electronic medical records were evaluated and the correlation of biomarkers and clinical symptoms was analyzed. Statistical analyses comprised Mann-Whitney-U test, Chi-square test and ROC analyses.

Results: AOSD patients had a median age of 40 years (range 8–85) and an observation period of 3.7 years (range 0.0–18.4). PP patients had a median age of 48 years (range 20–83) and an observation time of 2.2 years (range 0.0–13.9). Attack symptoms and medication showed distinct differences between the two groups. Approximately one third of patients had one to two attacks per year or even a chronic disease activity (≥3) while the other two-thirds had less than one attack per year in both groups.

C-reactive protein (CRP), serum amyloid A (SAA), sCLP and white blood cell count (WBC) correlated with the disease activity. The strongest correlation of biomarkers was found between CRP and SAA (AOSD r=0.823; PP r=0.769) and in relation to WBC the strongest correlation was detected between neutrophiles and sCLP (AOSD r=0.690; PP r=0.651).

Conclusion: sCLP, along with CRP and SAA, indicate systemic inflammation and correlate with disease activity in patients with AOSD and PP.