Immunoglobulin G4 Smoldering Multiple Myeloma With Immunoglobulin G4‐related Autoimmune Hepatitis: A Rare Case Report

Background Multiple myeloma (MM) is a common malignant tumor of the hematological system caused by the malignant proliferation of plasma cells, characterized by the production of M proteins and CRAB symptoms. Among them, the Immunoglobulin G MM is the most common, while the IgG4‐MM is extremely rare. Smoldering multiple myeloma refers to a state where there are no clinical symptoms. However, bone marrow plasma cell infiltration reaches 10%–59%, and previously, there were no reports of Immunoglobulin G4 smoldering multiple myeloma (IgG4 SMM) internationally. Case Presentation An over‐50‐year‐old woman visited the hospital due to abnormal liver function. Laboratory tests showed a significant increase in serum IgG4 (24.95 g/L), and serum protein electrophoresis detected IgG‐κ M protein (16.05 g/L). A liver biopsy showed IgG4 + plasma cell infiltration (11/HPF) and interface hepatitis. Bone marrow biopsy confirmed IgG4 monoclonal plasma cell proliferation. The diagnosis was IgG4‐κ SMM combined with Immunoglobulin G4‐related autoimmune hepatitis (IgG4‐AIH) and primary biliary cholangitis (PBC). The patient had no typical CRAB symptoms, and no osteolytic destruction was found in imaging. So we formulated a chemotherapy regimen using Bortezomib and dexamethasone, combined with azathioprine for immunomodulation. Unfortunately, after one session of chemotherapy, the patient did not return to the hospital for further evaluation. Conclusion This article explores the clinical features and diagnostic challenges of IgG4 SMM coexisting with IgG4‐AIH. IgG4 type SMM needs to be identified with IgG4‐RD. Clinicians should pay attention to IgG subtype detection and clonal plasma cell analysis.