Fig. 2. Flow cytometry analysis of three cases in which we were not able to detect malignant plasma cells using the conventional method (A-1, B-1, C-1: Plasma Cell Separation Tube (PCST) method; A-2, B-2, C-2: Conventional method). (A) The first case was a 56-year-old female patient who was diagnosed with kidney amyloidosis, and then diagnosed with multiple myeloma (MM) because of IgM-λ type monoclonal gammopathy (M protein: 0.02 g/dL), with increased plasma cells in bone marrow aspirates (27% of all the nucleated cells, ANCs). Immunophenotyping of plasma cells was CD19-CD56+CD45-/dim+. PCST was able to identify cytoplasmic lambda light-chain restriction. However, the conventional method failed to do so. (B) The second case was a 59-year-old female patient who was admitted with left flank pain. She was diagnosed as MM because of IgD-λ type monoclonal gammopathy (M protein: 0.51 g/dL) with increased plasma cells in bone marrow aspirates (35% of ANCs). Using flow cytometry analysis, plasma cells were found to represent 7.67% of ANCs, and 99.21% of these presented CD19-CD56dim+CD45-/dim+ immunophenotypes, whereas it was difficult to gate plasma cells by the conventional method, which prevented accurate analysis. (C) The third case was a 72-year-old female patient who was diagnosed with MM because of biclonal gammopathy, IgG-κ and IgG-λ (M protein: 2.03 g/dL), with increased plasma cells in bone marrow aspirates (22% of ANCs). Malignant plasma cells showed normal immunophenotypes, CD19+ CD56- CD45+. Therefore, malignant plasma cells could not be detected with either PCST or conventional methods. Additionally, cytoplasmic light-chain restriction could not be observed because of biclonal gammopathy composed of two different types of light-chains, κ and λ.

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