CD20 by diagnostic flow cytometry helps discriminate monoclonal gammopathy of undetermined significance and multiple myeloma plasma cells

Flow cytometry is a vital diagnostic tool for identifying clonal plasma cells in multiple myeloma (MM); yet the immunophenotype of non-MM monoclonal plasma cells remain incompletely defined. This is particularly important for monoclonal gammopathy of undetermined significance (MGUS) where early diagnostic discrimination from MM may assist early diagnosis and clinical decision-making. Over a 12-month period, all bone marrow plasma cell cases encountered in a single-centre diagnostic flow cytometry laboratory were retrospectively reviewed. Plasma cells were stained with a panel of antibodies consisting of CD45, CD38, CD138, CD19, CD20, CD56, CD117, intracellular kappa and intracellular light chains. Medical records were reviewed to classify cases as normal/polyclonal plasma cells, MM plasma cells and non-MM MGUS plasma cells. The review identified a total of 267 cases. Monoclonal plasma cells consistently demonstrated reduced or absent CD45 expression and loss of CD19 and CD20 compared with normal plasma cells. However, on comparing MGUS and MM cases, CD20 was more frequently retained in MGUS whereas it was typically absent in MM. These data suggest that CD20 may be a helpful marker in distinguishing MGUS from MM cases by flow cytometry. This work supports more accurate clinical classification of monoclonal plasma cell disorders and may ultimately improve better patient management.