Clinical utility and performance of anti-C1q antibodies for SLE: comparative analysis of three different assays

Objective Despite the reported clinical significance of anti-C1q immunoglobulin G autoantibodies (anti-C1q) for SLE and the development of anti-C1q commercial kits, their performance characteristics in routine patient evaluation remain poorly defined.Methods In this cross-sectional study, an unselected cohort of 107 patients with suspicion of a systemic autoimmune disease and 102 controls were included and evaluated for anti-C1q autoantibodies with three kits (Bühlmann, Orgentec and Werfen). Patients in the unselected cohort were classified as having SLE or a mimicker condition. Disease activity was assessed using Systemic Lupus Erythematosus Disease Activity Index (SLEDAI)-2K and SLE patients were subclassified based on the presence of lupus nephritis (LN) (history/active). Each kit was assessed for diagnostic performance. The degree of association between anti-C1q and SLEDAI/complement levels was assessed using the Spearman correlation coefficient (rho). Between-kit agreement was evaluated using Fleiss Ƙ.Results Werfen had the best performance in discriminating patients with SLE from non-SLE (area under the curve (AUC)=0.76), whereas for the SLE versus mimicker comparison Bühlmann (AUC=0.71) and Werfen (AUC=0.71) exhibited adequate discrimination. Among patients with SLE, all kits exhibited poor discrimination of patients with history of LN versus non-renal SLE, but Orgentec displayed good discrimination for patients with active LN from non-active LN (AUC=0.74). A positive correlation was observed between anti-C1q and SLEDAI scores, whereas a negative correlation was found between anti-C1q and complement C3 and C4 across all kits. Agreement for SLE between the three kits was moderate (Ƙ=0.42).Conclusion Anti-C1q can identify patients with SLE and correlate with global and renal disease activity, highlighting their potential to function as follow-up markers in SLE. Agreement with commercial anti-C1q kits is variable. Further studies are required to harmonise anti-C1q kits for optimal routine patient evaluation.