Renal biopsy was performed and showed 60% of glomeruli with glomerular basement membrane ruptures and crescent formation. The remaining glomeruli exhibited segmental mesangial cell proliferation and moderate basement membrane thickening. Direct immunofluorescence revealed diffuse global granular staining for IgG, IgA, IgM, and C3 along glomerular capillary walls. Electron microscopy identified subepithelial electron-dense deposits, with no significant subendothelial deposits (Fig. 1). The biopsy findings suggested overlapping features of lupus nephritis (LN) and ANCA-associated glomerulonephritis.

Initial treatment with intravenous methylprednisolone and cyclophosphamide yielded inadequate renal improvement, and the patient requested a prompt reduction in steroid dosage. This prompted the addition of belimumab in line with the latest KDIGO guidelines, alongside continued prednisolone, mycophenolate mofetil, and hydroxychloroquine. The patient achieved a partial renal response after 3 months of treatment, and this partial response has been maintained up to now, with 6 months of follow-up. The steroid dosage was reduced to 4 mg daily, with no significant changes in appearance or weight. Her 24-hour urine protein decreased to 0.9 g/d, serum creatinine normalized to 0.9 mg/dL, and ANCA, dsDNA, and ANA tests were negative, with serum complement levels returning to normal.

The coexistence of LN and ANCA-associated glomerulonephritis is rare, with unclear underlying causes potentially involving shared immune pathways or molecular mechanisms, and no established treatment protocols [1,2]. This case underscores the need to consider overlapping pathologies when conventional presentations do not fully explain clinical or histopathological findings and highlights the potential of tailored therapies like belimumab in managing complex cases.