Monoclonal gammopathy in systemic lupus erythematosus is associated with distinctive clinical course, malignancy and mortality rate: a single-centre retrospective cohort study.

Objectives: Rheumatic diseases were previously associated with increased incidence of monoclonal gammopathy (MG) and its malignant transformation. The present study aimed to investigate the prevalence, malignant transformation risk, clinical correlates and prognostic impact of MG in SLE.

Methods: A retrospective cohort study based on the medical records of n=1039 patients with SLE fulfilling the 1997 American College of Rheumatology (ACR), the 2019 European Alliance of Associations for Rheumatology (EULAR)/ACR and/or the 2012 Systemic Lupus International Collaborating Clinics (SLICC) criteria managed at two tertiary care departments of the University Hospital (Krakow, Poland) from January 2012 until November 2019.

Results: SLE+MG cases were older at SLE diagnosis compared with non-MG SLE controls (53±15 years vs 37±15 years, respectively, p<0.01), had higher rates of lymphopenia, anaemia, haemolysis, serous effusions and interstitial lung disease (all p<0.05), and were more frequently treated with cyclophosphamide (57% vs 28%, p<0.01) or rituximab (13% vs 3%, p<0.01). Most MG cases were detected within a year after SLE diagnosis (Q25, Q75: 0, 12 years). With the median follow-up of 11 years (Q25, Q75: 6, 19 years), 34.8% (8 cases) of the SLE+MG cohort were diagnosed with malignancy, compared with 8.1% (82 cases) among the SLE controls (p<0.001). MG was associated with the relative hazard of death of HR 2.99 (95% CI 1.26 to 7.06, p<0.05) and a median survival time from SLE diagnosis to death of 5 years (Q25, Q75: 1, 14; range 0-41) for SLE+MG cases, as compared with 12 years (Q25, Q75: 6, 19; range 0-62) for the controls. The effect was non-independent on antimalarial medication use.

Conclusions: Our study emphasises heightened malignancy and mortality rates in SLE+MG cases. The association between immunosuppression, MG incidence and progression warrants further research.