Background Complement levels are already known as biomarkers of flare in systemic lupus erythematosus (SLE); currently the usefulness of free light chains (FLC) in different autoimmune diseases in which the B cell has a relevant pathogenic role, as in the case of SLE, is being investigated.

Objectives To explore the usefulness of FLC determination as a flare biomarker in patients with SLE and to analyse possible discriminative differences between FLC and complement C3 and C4 levels.

Methods We performed an unicentric prospective longitudinal study with the following inclusion criteria: age greater than 18 years old and fulfilment of ACR or SLICC criteria for the diagnosis of SLE. Exclusion criteria were non-SLE related haematological disease, severe infection and severe kidney disease (Crea>2 mg/dl) to avoid interferences with FLC clearance. SLE flare definition was based on the SFI index. Receiver operator curves (ROC) and calculation of the area under the curve (AUC) were used to compare the discriminative ability between FLC and C3-C4 levels.

Results 46 patients were enrolled. For the present communication, only baseline data were analysed. 41 (91%) patients were women. Most frequent clinical manifestations were haematological (83%) and cutaneous (72%). Laboratory findings were 98% positive ANA, 67% positive antiDNAdc, 54% decreased C3% and 39% decreased C4. 6 (13%) patients presented a SLE flare and their characteristics compared to non-flare patients were lower C3 levels (70 vs 95; p=0,017) and C4 levels (10 vs 17; p=0,008) with higher concentrations of lambda light chains (λ LC) (27 vs 19; p=0,028). In addition, flare patients had higher IgA levels (402 vs 250; p=0,029), higher score in the global assessment of the doctor (4.3 vs 1.6; p=0,0003), longer time disease evolution (17.8 vs 14.3 years; p=0,844), higher values of SLEDAI (2.7±2 vs 1.4±1.3; p=0,083) and higher lymphopenia (982 vs 1474; p=0,068). We found no association between FLC levels and the presence of antiDNAdc. The correlation of FLC with the rest of activity biomarkers (C3, C4, SLEDAI, VAS) occurred in the expected way although the magnitude of the association was moderate, with a higher correlation between C3 and λ LC. λ LC were the only FLC that showed ability to discriminate flares (AUC 0.781), with sensitivity 100%, specificity 65% and 69.6% of patients correctly classified for a cut-off point ≥19.5. On the other hand, C3 and C4 also had a high discrimination capacity for flare (AUC 0.804 and 0.837 respectively). Comparing the three parameters, C4 achieved the highest AUC (0.837, 95% CI: 0.663–1000).

Conclusions Lambda free light chains have a good discrimination capacity for SLE flares and could be useful as a SLE flare biomarker. Longitudinal studies with a larger number of patients are necessary to evaluate its usefulness as a flare predictor.

Disclosure of Interest None declared