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Plasmablastic myeloma (PM) is a plasma cell neoplasm defined by Greipp, et al., in 1985 as multiple myeloma (MM) with a bone marrow infiltration ≥ 2% of plasmablasts. Even when PM is not considered an individual clinical entity in the 2016 World Health Organization classification of tumors of hematopoietic and lymphoid tissues, it has been associated with an increasingly aggressive and reduced survival, which was also evidenced in patients receiving stem cell transplantation.

PM is a high-risk disease. Therefore, its prompt identification is a relevant step during the diagnostic evaluation of MM. Plasmablasts are considered the most immature plasma cells and are usually characterized by a basophilic cytoplasm (Usually nucleus/cytoplasm ratio>0.6) with a faint or absent perinuclear hof, large nucleus (diameter>10μm), diffuse chromatin, and a very prominent nucleolus >2μm. PM should be differentiated from plasmablastic lymphoma (PL), a lymphoproliferative neoplasm, which also expresses CD38, CD138, MUM1, and high Ki67. PM expresses more frequently CD56, and PL is usually associated with higher expression of c-MYC rearrangements, HIV positive and EBER positive. In addition, the extramedullary disease with nodal involvement and ascites favors PL diagnosis, whereas the bone marrow involvement of plasma cells, with typical clinical characteristics of MM, favors PM.

Plasmablasts in panels A, B, and C (black arrow), mature aberrant plasma cells in panel D (white arrow), panel E presents a binucleated plasma cell, and panel F presents a Mott cell.