Determination of serum free light chains as a marker of systemic lupus flare.

ABSTRACT

INTRODUCTION: The aim of this study was to explore the usefulness of the determination of free light chains (FLC) in serum as a biomarker of flare in patients with systemic lupus erythematosus (SLE) and to analyze the differences in their discriminatory capacity with complement C3 and C4. METHODS: This was a prospective cohort study. The definition of flare was based on the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) Flare Index. The discriminatory capacity of FLC and C3 and C4 levels was compared using receiver operating characteristic (ROC) curves and the area under the curve (AUC). RESULTS: Forty-six patients were enrolled. Patients with SLE flare showed significantly lower C3 (p = 0.025) and C4 levels (p = 0.028), as well as a higher concentration of lambda light chains (λ-LC) (p = 0.028) compared with the non-flare group. λ-LC, as opposed to kappa light chains and total light chains, demonstrated a discriminatory capacity for detecting the presence of SLE flare (AUC 0.781), with 100% sensitivity, 65% specificity, and 69.6% of patients correctly classified for a cutoff point of ≥ 19.5 mg/L. Complement C3 and C4 also showed a high discriminatory capacity for SLE flare (AUC 0.804 and 0.837, respectively). Comparing λ-LC, C3, and C4, the last one demonstrates better discriminatory capacity for SLE flare with the highest AUC (0.837; 95% CI 0.663-1.000). CONCLUSIONS: λ-LC have good discriminatory capacity for SLE flare and could be useful as a biomarker of SLE exacerbation.Key Points• The usefulness of free light chains as a biomarker could be compared with complement.• Lambda free light chains have good discriminatory capacity for SLE flare.• Free light chains are a promising marker of SLE activity.