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

Introduction: Monoclonal gammopathy of undetermined significance (MGUS) is the most common plasma cell dyscrasia, defined by clonal monoclonal immunoglobulin (MIg) in serum and/or urine in the absence of end-organ damage caused by plasma cell proliferation. By contrast, monoclonal gammopathy of renal significance (MGRS) encompasses a spectrum of kidney disorders directly or indirectly driven by MIg, with clinical phenotypes ranging from insidious proteinuria to rapidly progressive glomerulonephritis. The diagnostic ambiguity arising when MGUS coexists with renal dysfunction remains a major clinical challenge. Here, we report a rare case of a patient initially suspected to have MGRS complicated by pauci-immune crescentic glomerulonephritis (PICGN). However, this diagnosis was ultimately excluded by the absence of MIg deposits in renal tissue on histopathological examination. Case Presentation: We describe a female with concurrent PICGN and MGUS, 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 MIg 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. Conclusions: 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 ANCA-associated vasculitis and monoclonal gammopathies, particularly in cases with overlapping renal injury features.