TOPLINE:

Serum calprotectin and complement factor 3 (C3) levels were elevated in treatment-naive patients with early psoriatic arthritis (PsA) and declined with treatment; these markers outperformed C-reactive protein (CRP) in detecting systemic inflammation among patients who had low or normal CRP levels or obesity.

METHODOLOGY:

• As part of a prospective observational study, researchers evaluated alternative blood biomarkers of inflammation in patients with early PsA at two Belgian rheumatology centres from October 2017 to December 2020.
• They included 67 adult patients with early PsA who were not treated with either conventional synthetic or biological disease-modifying antirheumatic drugs (mean age, 47.8 years; 70.1% men). They were compared with 61 matched healthy control individuals and 50 patients with early rheumatoid arthritis (RA).
• Subgroups of patients with PsA who had obesity (40.3%) and those who were CRP negative (58.2%) were analysed. Obesity was defined as a BMI of 30 or above, and CRP levels of 5 mg/L or less were considered CRP negative.
• Patients were evaluated at baseline and after 1 year for levels of various components, including CRP, serum calprotectin, and C3.
• An exploratory composite score was calculated using C3 and calprotectin levels and was used to assess the diagnostic performance in a post hoc analysis.

TAKEAWAY:

• C3 and calprotectin levels were consistently higher among patients with early PsA than among healthy control individuals (median C3, 1.3 vs 0.95 g/L; median calprotectin, 1867 vs 1175 mg/mL; P < .001 for both), and their levels were comparable to those among patients with early RA.
• Among patients who were CRP negative, both C3 and calprotectin levels remained significantly elevated in those with PsA or RA compared with those in healthy control individuals (P < .001 for both).
• In patients with obesity, a composite marker of C3 and calprotectin demonstrated superior diagnostic performance than CRP (area under the curve, 0.836 vs 0.718; sensitivity, 76%; specificity, 72.7%).
• C3 and calprotectin levels decreased significantly after 1 year in patients with PsA who achieved low disease activity (P < .01), supporting their utility in monitoring.

IN PRACTICE:

"[The study] findings support the value of C3 and calprotectin as biomarkers of systemic inflammation in PsA," the authors wrote.

"These markers likely reflect underlying pathophysiology more accurately than traditional acute-phase reactants and may reflect disease activity in early PsA," they added.

SOURCE:

This study was led by Alla Ishchenko, UZ Leuven, Leuven, Belgium. It was published online on September 23, 2025, in RMD Open.

LIMITATIONS:

The sample size was modest, and the investigators evaluated a limited panel of inflammatory markers. Markers were not tested in other rheumatic conditions or in patients with psoriasis alone; hence, they may not be used as diagnostic tools.

DISCLOSURES:

This study received funding from the Innovative Medicines Initiative 2 Joint Undertaking that receives support from the European Union's Horizon 2020 research and innovation programme and European Federation of Pharmaceutical Industries and Associations. Several authors reported receiving fellowships and grants from multiple sources, including PARTNER, GRAPPA, and Flanders Research Foundation. Three authors reported receiving consultancy and speaker fees and research grants from multiple pharmaceutical and healthcare companies and other sources.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.