Atypical Localization of Malignant Plasma Cells in Non-Viable Cell Area on Flow Cytometry Light-Scatter Dot Plot.

BACKGROUND Multi-parameter (multicolor) flow cytometric study of the bone marrow aspirate is a very useful tool for diagnosis of plasma cell dyscrasia and for evaluation of post-therapy bone marrow for minimal residual disease. CASE REPORT We present a case of a 50-year-old man with multiple myeloma, whose plasma cells on a bone marrow aspirate flow cytometric study showed atypical placement on a light-scatter dot plot, both on forward and side scatter. The bone marrow aspirate sample was 33 hours and 11 minutes old, and the light-scatter dot plot demonstrated that plasma cells, detected by their expression of CD138, CD38, and CD56, occupied an area otherwise characteristic for dead cells and cell detritus. Expressions of CD138 and CD56 were dim (down-regulated). CONCLUSIONS Morphologically atypical plasma cells with irregular nuclear contours/polylobated nuclei from non-fresh samples can present with atypical localization in the area of dead cells. Our study of the multiple myeloma patient with normal localization of plasma cells on a light-scatter dot plot showed a fraction of plasma cells in the dead cell area with dim expression of CD138 and CD56, suggesting that plasma cells may deteriorate (age) rather rapidly, losing surface markers even in less than 24-hour-old specimens. We suggest that the non-viable cell/dead cell area should be checked for expression of CD138 so as not to miss plasma cell dyscrasia, especially if the specimen was run 24 hours after bone marrow sampling.