Biclonal gammopathy with coexistent proximal tubulopathy and cast nephropathy in multiple myeloma: a rare case report

BACKGROUND: Multiple myeloma is a hematological malignancy originating from neoplastic plasma cells, typically characterized by the presence of a monoclonal immunoglobulin. Light chain cast nephropathy is the most common cause of acute kidney injury in myeloma patients, typically presenting with elevated serum creatinine. Light-chain proximal tubulopathy is a manifestation of monoclonal light chain-associated renal injury, characterized by the accumulation of free light chains within proximal tubular cells. However, the co-occurrence of κ light-chain MM with an additional IgG-λ monoclonal band, cast nephropathy with normal serum creatinine, and light-chain proximal tubulopathy is extremely rare, posing significant challenges to diagnosis and treatment. This case report aims to enrich the clinical understanding of such rare comorbidities. CASE PRESENTATION: A 54-year-old female was admitted due to "detection of urinary protein for 2 months". Laboratory tests showed massive proteinuria, mild anemia, normal serum creatinine, and significantly abnormal serum free light chains. Serum immunofixation electrophoresis revealed monoclonal IgG-λ and free κ chains, while urine immunofixation electrophoresis only showed free κ chain positivity. Renal biopsy confirmed κ-type cast nephropathy, with irregular lysosomes in proximal tubular epithelial cells and dominant κ light chain expression. Bone marrow examinations indicated plasma cell myeloma. The final diagnosis was the final diagnosis was confirmed as κ light-chain multiple myeloma with an additional IgG-λ monoclonal band, complicated with cast nephropathy and light-chain proximal tubulopathy. After two courses of chemotherapy, urinary protein and serum free light chain levels improved significantly, but hemoglobin decreased to severe anemia. Subsequent bone marrow aspiration indicated severe erythroid hypoplasia, then the treatment was adjusted, and the hemoglobin level improved subsequently. CONCLUSIONS: This rare case demonstrates that multiple myeloma can harbor two clones and can present with concurrent cast nephropathy and light chain proximal tubulopathy, even when serum creatinine remains normal. Additionally, the transient improvement followed by severe erythroid hypoplasia after chemotherapy suggests the need for close monitoring of hematopoietic complications. This case expands the clinical spectrum of biclonal gammopathy and underscores the value of comprehensive pathological evaluation for accurate diagnosis and tailored management.