Monoclonal immunotactoid glomerulopathy with masked IgG associated with symptomatic multiple myeloma: a case report diagnosed by paraffin-immunofluorescence and mass spectrometry.

Immunotactoid glomerulopathy (ITG) is a rare glomerulonephritis characterized by the deposition of hollow-centered microtubular structures on electron microscopy (EM), and by IgG, C3, and restricted light chain deposition on immunofluorescence (IF) staining. It is often associated with hematological malignancies, including lymphoma and lymphocytic leukemia. A 71-year-old Japanese woman presented with increased proteinuria (urine protein-to-creatinine ratio, 4.32 g/gCr) and impaired kidney function (serum creatinine, 1.24 mg/dL). A kidney biopsy was performed, and routine frozen-IF revealed positivity for C3 and κ-light chain along the glomerular basement membrane, while IgG was negative. EM demonstrated electron-dense deposits composed predominantly of microtubules with hollow centers measuring 15-20 nm in diameter and arranged in parallel arrays, raising suspicion of ITG. Mass spectrometry and paraffin-IF following pronase K pretreatment detected IgG2 along the capillary walls, leading to the diagnosis of monoclonal ITG with masked IgG. In addition, the presence of paraproteins in serum and urine prompted bone marrow examination, which confirmed symptomatic multiple myeloma. With chemotherapy, the M-protein level decreased, proteinuria improved to less than 0.5 g/gCr, and renal function remained stable. We describe a rare case of monoclonal ITG with masked IgG, diagnosed by mass spectrometry and paraffin-IF, in a patient with symptomatic multiple myeloma. Early diagnosis and therapy for multiple myeloma stabilized renal function and reduced proteinuria. When a discrepancy between IF and EM findings is observed, especially when false-negative immunoglobulin staining is suspected, restaining with paraffin-IF or evaluation by mass spectrometry should be considered to ensure accurate diagnosis and appropriate treatment.