Background/Purpose: Psoriatic arthritis (PsA) is an inflammatory arthritis that occurs in a subgroup of patients suffering from psoriasis. Assessment of PsA disease activity currently mainly rely on clinical examination of the patient, global scales of pain and activity, and the health assessment questionnaire. However, as in other inflammatory arthritides, there is a scarcity of biomarkers for disease activity, with erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) being relatively unspecific and within normal limits in many occasions. Serum complement C3 component has recently been suggested as a potential biomarker for disease activity in PsA. Objective:  To explore the association of complement C3 component with disease activity in PsA.

Methods: We included 36 outpatients fulfilling the CASPAR classification criteria for PsA in our study. Patients were examined at baseline (n = 36), at week 12 (n = 22), and at week 24 (n = 13). Additionally to routine medical examinations, each patient received a full workup including patients and physicians global assessment (VAS), amount of pain (VAS), 66/68 joint count, skin involvement (% of body surface area), enthesitis assessment (Leeds Enthesitis Index), dactylitis count, the SF-36 questionnaire and a blood examination (white blood count, CRP, ESR, Complement C3c, Complement C4). As a surrogate marker for disease activity measurement we calculated the Psoriatic ArthritiS Disease Activity Score (PASDAS). C3c and C4 complement were measured by nephelometry out of heparin-plasma samples of the patient. We performed correlation analysis of the C3c and C4 components with individual markers of disease activity (see above) and then performed unvariable and adjusted regression analysis on the PASDAS. The statistical analysis was conducted using SPSS statistics.

Results: Pearson’s correlation showed a significant linear correlation of C3c (p = 0.001, r = 0.372) and C4 (p = 0.048, r = 0.236) with the patients PASDAS (see table for individual associations of PsA manifestations). In regression analysis, after correction for age, BMI and duration of disease, the correlation of C3c and PASDAS remained significant (p = 0.004, R squared = 0.155), while there was no statistical significance in the correlation of C4 and PASDAS.

Conclusion: In this pilot study in patients with PSA, we observed a significant and independent association of serum C3c with overall disease activity (PASDAS). Looking at individual measures of PsA manifestations, associations for serum C3c were seen for patient global assessment, physicians global assessment, swollen joint count, skin involvement, ESR, CRP and SF-36 physical component score. These findings warrant further investigation of the usefulness of C3c complement component in disease activity monitoring of PsA.