BACKGROUND: Numerous studies have reported on the immunophenotype of plasma cells (PCs) in monoclonal gammopathy of undetermined significance (MGUS) and in plasma cell myeloma (PCM), but very few have examined the immunophenotype of normal PCs. In this study, an objective definition of normal range of expression for each antigen was found on normal control PCs. Using these new ranges of normal expression (new method) is different from using a static 20% of PCs cut-off for all antigens as described in the literature (traditional method). These newly calculated normal ranges for each antigen were applied to our data, and compared to histologic and immunohistochemical findings. METHODS: Bone marrow samples from 46 patients with PC neoplasms and 15 normal controls were studied. A minimum of 100 PC were analyzed for each patient and control sample. An 8-color staining method was applied to study the immunophenotype of PCs, using a BD FACSCanto II. RESULTS: By the new ranges of normality calculated in this study it was determined that different antigens have different level of expression on polyclonal PCs. CD19 correlated with histology by both the traditional and new methods, but had superior correlation by the new method. CONCLUSIONS: This report is the first 8-color immunophenotypic study of PCM in which a "range of normal expression" for each antigen is defined. This is a critical step to help distinguish between a normal and neoplastic PC immunophenotype and discern which antigens are of diagnostic importance.
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