ArticleFrontiers in medicine2026
Case Report: Diagnostic uncertainty in IC-MPGN coexisting with IgM-κ MGUS.
Article in Frontiers in medicine, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
Background: Distinguishing immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) with coincident monoclonal gammopathy of undetermined significance (MGUS) from monoclonal gammopathy of renal significance (MGRS) remains challenging when biopsy sampling is inadequate and gold-standard pathologic workup is incomplete. Case Presentation: A 60-year-old woman presented with upper abdominal fullness and heartburn. Routine testing revealed nephrotic-range proteinuria, acute kidney injury (AKI), and hypocomplementemia. Serum immunofixation showed IgM-κ monoclonal gammopathy. Bone marrow examination identified a clonal B-lymphocyte population of approximately 1% with no evidence of lymphoplasmacytic lymphoma or multiple myeloma. Kidney biopsy demonstrated IC-MPGN type I with mesangial and subendothelial electron-dense deposits. Immunofluorescence revealed granular deposition of IgG, IgM, C3, kappa, and lambda, indicating a polyclonal immune complex pattern. Cryoglobulin testing was negative on three occasions under strict 37 °C conditions. These findings suggested but could not confirm IC-MPGN with coincident IgM-κ MGUS; MGRS could not be excluded because only one non-sclerotic glomerulus was available for light microscopy and pronase-digested paraffin immunofluorescence was not performed. The patient received methylprednisolone pulse therapy followed by rituximab. At 1-month follow-up, serum albumin increased from 25.6 to 31.3 g/L, serum creatinine decreased from a peak of 231.0 to 94.0 μmol/L, and 24-h urine protein decreased from 7,930 mg to 1.1 g. Complement C3 and C4 normalized. Clinical improvement was observed following combined therapy; however, the independent contribution of rituximab cannot be determined from this single case. Conclusion: This case underscores the diagnostic uncertainty inherent in distinguishing coincident MGUS from MGRS when polyclonal immunofluorescence coexists with monoclonal gammopathy but gold-standard pathologic workup is incomplete. It highlights the need for standardized pathologic evaluation, including pronase-digested paraffin immunofluorescence, and illustrates that routine urinalysis in older adults with non-renal symptoms can reveal clinically significant kidney disease.
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