Supplementary Material for: MGRS or MGUS? Diagnostic Dilemma of ANCA-Associated Pauci-Immune Crescentic Glomerulonephritis with Monoclonal Gammopathy: A Case Report

This case report presents a 55-year-old female with concurrent pauci-immune crescentic glomerulonephritis (PICGN) and monoclonal gammopathy of undetermined significance (MGUS), highlighting the diagnostic challenges in differentiating between monoclonal gammopathy of renal significance (MGRS) and MGUS in the context of ANCA-associated vasculitis (AAV). The patient presented with acute kidney injury, proteinuria, and hematuria, with renal biopsy revealing type III crescentic glomerulonephritis and immunofluorescence showing weak positivity for κ and λ monoclonal immunoglobulin deposits. Despite initial suspicion of MGRS, immuno-electron microscopy did not confirm monoclonal light chain deposition, leading to a final diagnosis of PICGN and MGUS. The patient was treated with glucocorticoids and cyclophosphamide, resulting in stabilization of renal function but necessitating maintenance hemodialysis due to irreversible renal damage. This case underscores the importance of integrating serological, histopathological, and advanced imaging techniques to distinguish between autoimmune and plasma cell dyscrasias in renal pathology. It also emphasizes the limitations of immunofluorescence alone in diagnosing MGRS and the necessity of immuno-electron microscopy for definitive exclusion. This report calls for further research into the pathophysiological interactions between AAV and monoclonal gammopathies, particularly in cases with overlapping renal injury features.