Detection of myeloma cells in the peripheral blood by flow cytometry

Bone marrow plasma cells (PC) from patients with multiple myeloma (MM) express monoclonal cytoplasmic immunoglobulin (clg) light chain, strongly express CD38, and usually lack or dimly express CD45. The detection of malignant plasma cells in the peripheral blood (PB) by immunofluorescence microscopy (IM) distinguishes patients with active MM from those with stable disease. The aim of this study was to learn whether two-color (CD38 and CD45) flow cytometry (FC) on whole blood specimens (WBFC) and three-color FC (CD38, CD45, and anti-kappa or lambda clg) on mononuclear cells could identify circulating PC as well as the standard, more labor intensive IM technique. Split-samples of PB from 73 patients with plasma cell proliferative disorders were examined by both techniques. WBFC detected CD38+ CD45- cells in 94% (33/35) of patients with circulating monoclonal PC detected by IM and three-color FC detected monoclonal CD38+ CD45- cells in 77% (27/35) of these cases. The absolute number of monoclonal PC detected by IM was compared to the FC methods and the Spearman rank correlations were 0.77 with WBFC and 0.80 with three-color FC. This study indicates that WBFC, using antibodies to CD38 and CD45, offers a practical and reliable method to detect and quantify circulating malignant PC in patients with MM.