Objective. SLE is an autoimmune disorder characterized by abnormal complement activation. Numerous new biomarkers have recently been used to diagnose or monitor disease activity in patients with SLE. We checked the levels of erythrocyte-bound C4d (E-C4d), an activation-derived fragment of C4 that is deposited on the erythrocytes, under different conditions of SLE in order to correlate these levels with disease activity.

Methods. We conducted a cross-sectional investigation of three groups of patients: (i) 63 patients with SLE; (ii) 43 patients with other diseases; and (iii) 26 healthy controls. Erythrocytes were analysed by flow cytometry to determine the levels of E-C4d.

Results. We found a significant elevation in the mean levels of E-C4d in SLE patients compared with patients with other diseases or healthy controls. In SLE patients, the levels of E-C4d were correlated with the SLEDAI and inversely correlated with serum C3/C4 levels. In the subgroup of SLE patients with haemolytic anaemia (HA), a significantly higher level of E-C4d was observed than that in SLE patients without HA. However, in SLE patients with HA, there was no correlation between the levels of E-C4d and other markers of disease activity, including SLEDAI and levels of anti-dsDNA, C3 and C4.

Conclusion. E-C4d levels are useful diagnostic markers for SLE and can serve as biomarkers of disease activity in patients with SLE. However, E-C4d is of limited value in monitoring disease activity in SLE patients with HA.