Clinical Significance of Overlap Syndrome of Histologically Confirmed Lupus Nephritis with Antineutrophil Cytoplasmic Antibody-Associated Vasculitis

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Objectives: We applied the 2022 American College of Rheumatology/ European Alliance of Association for Rheumatology (ACR/EULAR) criteria for antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) to patients histologically diagnosed with lupus nephritis (LN) to investigate the overall rate of and initial contributing factors to the reclassification of overlap syndrome of LN with AAV (OS-LN-AAV). Methods: We retrospectively reviewed the medical records of 1292 patients with systemic lupus erythematosus (SLE) and included 164 patients with LN in this study. Patient demographics, SLE manifestations, LN classes, and laboratory data, including ANCA levels, were recorded. All-cause mortality and end-stage kidney disease (ESKD) were evaluated as poor outcomes. Results: The median age of the 164 patients was 37.0 years, and 12.2% were men. The overall reclassification rate was 37.8%, of which 34.1% and 3.7% of the patients were reclassified as having OS-LN-microscopic polyangiitis and OS-LN-granulomatosis with polyangiitis (GPA), respectively, but none as having eosinophilic GPA. ANCA positivity and AAV-suggesting lung lesions were major contributors to OS-LN-AAV reclassification. When patients were compared based on OS-LN AAV reclassification, ANCA positivity and myeloperoxidase-ANCA (or P-ANCA) positivity favoured for OS-LN-AAV reclassification, whereas oral ulcers did not. However, OS-LN-AAV reclassification did not affect all-cause mortality or ESKD. Conclusions: This is the first study demonstrating a 37.8% reclassification rate in patients histologically diagnosed with LN using the 2022 ACR/EULAR criteria for AAV. Furthermore, it was also the first to reveal ANCA positivity and AAV-suggesting lung lesions as major contributors to OS-LN-AAV reclassification.