A GEIL flow cytometry consensus proposal for quantification of plasma cells: Application to differential diagnosis between MGUS and myeloma

Background: Flow cytometry is the sole available technique for quantification of tumor plasma‐cells in plasma‐cell disorders, but so far, no consensus technique has been proposed. Here, we report on a standardized, simple, robust five color flow cytometry protocol developed to characterize and quantify bone marrow tumor plasma‐cells, validated in a multicenter manner. Methods: CD36 was used to exclude red blood cell debris and erythroblasts, CD38 and CD138 to detect plasma‐cells, immunoglobulin light chains, CD45, CD56, CD19, and CD117 + CD34 to simultaneously characterize abnormal plasma‐cells and quantify bone marrow precursors. This approach was applied in nine centers to 229 cases, including 25 controls. Results: Tumor plasma‐cells were detected in 96.8% of cases, all exhibiting an immunoglobulin peak over 1g/L. Calculation of a plasma‐cells/precursors (PC/P) ratio allowed quantification of the plasma‐cell burden independently from bone marrow hemodilution. The PC/P ratio yielded the best results in terms of sensitivity (81%) and specificity (84%) for differential diagnosis between MGUS and myeloma, when compared with other criteria. Combination of both the PC/P ratio and percentage of abnormal plasma‐cells allowed the best differential diagnosis, but these criteria were discordant in 25% cases. Indirect calculation of CD19 negative PC/R ratio gave the best results in terms of sensitivity (87%). Conclusion: This standardized multiparameter flow cytometric approach allows for the detection and quantification of bone marrow tumor plasma‐cell infiltration in nearly all cases of MGUS and myeloma, independently of debris and hemodilution. This approach may also prove useful for the detection of minimal residual disease. © 2010 International Clinical Cytometry Society