Bone marrow aspirate enrichment procedure effectively depletes nontumor cells. (A) Bone marrow aspirate samples before and after enrichment were subject to CD138 staining and FACS analysis. (B) Myeloma purity score is elevated in control plasma cell samples (> 90% pure) relative to bone marrow mononuclear cells (MNCs), neutrophils, and erythroid cells. Two enriched patient samples of 84% and 91% tumor purity by FACS analysis had scores of 35 and 28, respectively (blue arrows). A score of at least 10 (at least 3-fold elevated relative to the score for nonplasma cell types) was set as a threshold for further analysis.