Establishment of a Time-Resolved Fluoroimmunoassay for sC5b-9 and Validation in Lupus Nephritis

This study aimed to develop a time-resolved fluoroimmunoassay (TRFIA) for the quantitative detection of soluble terminal complement complex (sC5b-9) and to evaluate the clinical significance of urinary sC5b-9 in patients with lupus nephritis (LN). The sC5b-9-TRFIA was established using a 96-well microplate coated with anti-sC5b-9 monoclonal antibody, anti-sC5b-9 polyclonal antibody, and Eu³⁺-labeled goat anti-rabbit IgG. Urinary sC5b-9 levels were determined in 42 patients with LN and 18 healthy controls to further assess its diagnostic potential in LN. The assay demonstrated a detection limit of 1.38 ng/mL and a linear measurement range of 6.25-1000 ng/mL. The intra-assay and inter-assay coefficients of variation ranged from 4.84% to 7.91% and from 7.06% to 9.11%, respectively, indicating good precision. The sC5b-9-TRFIA showed strong agreement with commercially available enzyme-linked immunosorbent assay (ELISA) kits within the same detection range (P < 0.0001). Urinary sC5b-9 concentrations were significantly elevated in LN patients (157.45 ± 127 ng/mL) compared with healthy controls (38.87 ± 30.11 ng/mL, P < 0.0001). In summary, a wide-range sC5b-9 TRFIA based on europium (Eu³⁺) chelate labeling was successfully developed, providing a sensitive and reliable method for assessing the clinical condition of LN.