Serum calprotectin and complement factor C3 are superior biomarkers of inflammation in early psoriatic arthritis as compared with C-reactive protein.

Objectives: C reactive protein (CRP) is frequently normal in psoriatic arthritis (PsA) despite active disease, complicating inflammation assessment. This study aimed to evaluate alternative biomarkers of inflammation in early PsA.

Methods: Adult patients with early, treatment-naïve PsA were enrolled in the prospective multicentre cohort and compared with early rheumatoid arthritis (RA) and healthy controls (HCs). Clinical assessments, inflammatory markers and peripheral blood counts were collected. For this study, baseline and 1-year data were used. Serum complement factor 3 (C3), calprotectin (S100A8/9) and serum amyloid A (SAA) were measured by ELISA. Discriminatory performance was evaluated using receiver operating characteristic curve analysis.

Results: A total of 67 PsA, 50 RA patients and 61 HC were included. At baseline, median levels of C3 (1.38 g/L) and S100A8/9 (5.58 µg/mL) were significantly elevated in PsA compared with HC and were comparable to RA. In the 'CRP-negative' subgroup, C3 and S100A8/9 were increased in PsA as compared with HC. In the obese subgroup, CRP levels did not discriminate PsA, RA and HC. However, S100A8/9 was significantly increased in PsA and RA as compared with HC, whereas SAA and derived inflammatory ratios (neutrophil/monocyte ratio, lymphocyte/monocyte ratio) did not discriminate PsA, RA or HC. After 1 year, C3 and S100A8/9 decreased significantly in PsA patients achieving low disease activity. In the obese subgroup, the composite marker C3×calprotectin demonstrated superior diagnostic performance as compared with CRP (area under the curve=0.836).

Conclusions: C3 and calprotectin are elevated in early PsA and are responsive to treatment. These markers outperform CRP in obese and CRP-negative patients and may support improved diagnosis and disease monitoring in clinical practice.