[Immunoglobulin D lambda multiple myeloma: diagnosis guided by light chain deposition nephropathy]

Immunoglobulin D (IgD) multiple myeloma is a rare entity, accounting for 1 to 2% of myelomas, characterized by aggressive progression and frequent renal involvement. Its diagnosis is often delayed due to the low serum concentration of IgD and the absence of a monoclonal peak on serum protein electrophoresis. We report the case of a 56-year-old patient with no significant medical history who was hospitalized for severe renal failure associated with profound anemia. Renal biopsy revealed myelomatous nephropathy with tubular deposits of lambda light chains on immunofluorescence. Serum protein electrophoresis revealed hypogammaglobulinemia with an increase in the beta-2-globulin fraction. Serum immunofixation identified a monoclonal IgD lambda immunoglobulin associated with monoclonal free light chains (FLC) of lambda isotype. Serum FLC measurement confirmed a massive excess of lambda chains. The diagnosis was made in accordance with the 2014 International Myeloma Working Group (IMWG) criteria. This observation illustrates the diagnostic pitfalls of this entity and highlights the major importance of extended immunofixation, serum FLC measurement, and renal biopsy for early identification.