Abstract
IgM multiple myeloma (MM) is an extremely rare lymphoproliferative disease associated with an aggressive clinical course. However, the diagnosis of IgM MM may be complicated by Waldenstrom's macroglobulinemia (WM), particularly when clinical manifestations and morphological features are not typical. It is crucial to distinguish between IgM MM and WM as their prognoses and treatment strategies are different. We report a case of IgM MM presenting with bleeding tendency and an immunophenotype analysis using flow cytometry and immunohistochemistry. Bone marrow cells exhibited a typical phenotype for plasma cells, expressing monoclonal cytoplasmatic IgL-λ, CD38 and CD138 instead of pan-B cell antigens CD19, CD20 and CD22, which are characteristic of the typical immunophenotype of WM. Therefore, the diagnosis of IgM MM was confirmed in this case, highlighting the significance of detailed immuno-phenotypic evaluation when clinical and morphological features are atypical.