Early-stage IgM kappa proliferative glomerulonephritis with monoclonal immunoglobulin deposition presenting with atypical renal insufficiency: a case report and literature review.

INTRODUCTION: Proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) is a rare subtype of monoclonal gammopathy of renal significance (MGRS). While the IgG subtype has been well characterized, the IgM-κ variant is extremely rare, with a low detection rate of circulating monoclonal proteins and limited real-world data on its clinical course under conservative management. This case report describes a biopsy-proven IgM-κ PGNMID in an elderly patient with negative hematological screening, to expand the current understanding of this rare entity. PATIENT CONCERNS AND DIAGNOSES: A 70-year-old male presented with incidentally detected microscopic hematuria and elevated serum creatinine, without overt edema, gross hematuria or systemic symptoms. Renal biopsy demonstrated a membranoproliferative pattern of glomerular injury, with monoclonal IgM deposition showing kappa light chain restriction and negative C3 staining, confirming the diagnosis of PGNMID. Bone marrow biopsy revealed no clonal lymphoplasmacytic proliferation, consistent with MGRS. The patient's serum creatinine level remained stable with a slight decrease from the baseline value, and the urinary red blood cell count returned to normal. OUTCOMES: After 7 months of follow-up, the patient's serum creatinine levels remained stable with a mild decrease from baseline. This case indicates that IgM-κ PGNMID can present with insidious manifestations and negative conventional hematological screening. For elderly patients with unexplained hematuria and mild renal impairment, early renal biopsy is essential for early identification of rare MGRS lesions, and conservative management may be considered as an initial approach in PGNMID patients with stable mild disease.