Free Light Chain Multiple Myeloma: Atypical Appearance on Serum Immunofixation.

Light chain multiple myeloma (LCMM) is a malignant hematological disease characterized by bone marrow infiltration by tumor plasma cells and the secretion of monoclonal free light chains (κ or λ). It is often di-agnosed through hypogammaglobulinemia detected by serum protein electrophoresis, followed by immunotyping showing a monoclonal band in free light chains. However, the structure of monoclonal light chains can sometimes complicate laboratory findings. The case studied involves a 52-year-old female patient hospitalized for diffuse bone pain and hypercalcemia. Capillary serum protein electrophoresis revealed hypogammaglobulinemia, leading to serum immunofixation (IFE). IFE showed a wide additional band in κ light chains, with no correspondence to the heavy chain antisera (GAM, D, and E), but similar reactivity with the anti-κ light chain, creating an interpretive challenge. The free light chain assay revealed a very high κ/λ ratio (1,068). Bone marrow examination showed 85% plasma cell infiltration, confirming the diagnosis of LCMM. This case highlights the diagnostic challenges encountered in certain cases of light chain multiple myeloma and underscores the importance of a multimarker approach combining serum protein electrophoresis, serum immunofixation, and free light chain assays.