Plasma cell
Plasma cell leukemia is a form of multiple myeloma in which significant numbers of typically immature appearing plasma cells, i.e. plasmablasts, circulate in the blood. Very small numbers of plasma cells may reach the circulation in non-IgM multiple myeloma, non-IgM SMM, and, exceptionally, non-IgM MGUS. In these plasma cell dyscrasias, the presence of even very small numbers of circulating plasma cells is a poor prognostic indicator. In plasma cell leukemia, however, circulating plasma cells reach far higher numbers and at these circulating levels are associated with exceptionally poor survival rates. The International Myeloma Working Group has defined the diagnostic criteria for plasma cell leukemia as the presence in blood of &gt;2x109 plasma cells per liter or, alternatively, &gt;20% of nucleated blood cells being plasma cells. More recently, the Group has suggested that values of 0.5x109 or 5%, respectively, may be more appropriate from a therapeutic viewpoint and therefore should be studied as a definitive criterion for the disease. A recent study supported this suggestion in finding that multiple myeloma patients with &gt;5% circulating plasma cells had a prognosis much worse than that for multiple myeloma and similar to that for plasma cell leukemia. Flow cytometry immunophenotyping of blood cells to detect clonal phenotypes of plasma cells seen in multiple myeloma (e.g. the CD138+, CD38+, CD19−, CD4+/- phenotype) may be a more sensitive method to enumerate circulating clonal plasma cells and diagnose plasma cell leukemia.