Direct measurement of CD34+ blood stem cell absolute counts by flow cytometry

For the collection of adequate numbers of peripheral blood stem cells (PBSC) for PBSC transplantation, an accurate quantification of circulating CD34+ stem cells is required for deciding the optimal time of the collection. To enumerate peripheral blood (PB) CD34+ stem cells, the percentage of CD34+ cells in the gated PB mononuclear cells should be multiplied by the percentage of the gated mononuclear cells among white blood cells (WBC) and by the total WBC count. Accordingly, a minor difference in the measured percentage of the CD34+ cells can lead to a major difference in the PB CD34+ cell concentration. In the present study, we measured the concentration of PB CD34+ stem cells with a flow cytometer designed to provide direct absolute counts of cell subsets from a single instrument. Whole blood was stained with a phycoerythrin-conjugated anti-CD34 monoclonal antibody, and, after the lysis of red blood cells, CD34+ cells were counted in a fraction of the lymphocyte and monocyte gate. The accuracy of our method was demonstrated in an experiment in which various dilutions of known numbers of CD34+ leukemic cells were mixed with normal blood; the predicted value of the CD34+ cell count was observed. The concentration of CD34+ cells in leukapheresis products was measured both by our direct assay and an indirect assay that calculates the number from the percentage of CD34+ cells in mononuclear cells, and our assay was shown to produce less variation. Further, our assay showed a significant correlation between the concentration of mobilized CD34+ cells in the PB and the number of harvested CD34+ cells in leukapheresis. These findings indicate that the monitoring of the concentration of PB CD34+ cells by the present method can be used to predict the number of stem cells collected in leukapheresis. This procedure is easy to perform and can be applied to daily monitoring to decide the appropriate timing for harvest of mobilized stem cells.