Minimization of CD34+ Cell Enumeration Variability Using the ProCOUNT Standardized Methodology

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Abstract
The dose of cells expressing the surface antigen CD34 (CD34+) has been shown to be a reliable predictor of the time to engraftment following transplantation of PBPC to support high-dose chemotherapy. However, evaluation of rare cells is complicated by a number of factors, including the variability in operator and technical procedures. Recently, Becton Dickinson Immunocytometry Systems introduced a new CD34+ cell analysis system, the ProCOUNT cell enumeration kit, which automates the analysis of CD34+ cells and minimizes the variabilities of this procedure. We have evaluated the ProCOUNT system in comparison to a standard CD34 cell analysis (based on the Milan approach) using leukapheresis products from patients and normal donors mobilized with chemotherapy plus recombinant human G-CSF (rhG-CSF) or with rhG-CSF alone. In addition, we compared these analyses using CD34+ cell-selected mobilized leukapheresis products with purities of 75% or greater. The standard CD34 cell analysis methodology quantitated the frequency of cells identified as CD45+, low side scatter, and CD34+. A high correlation coefficient was obtained between the ProCOUNT methodology and the standard CD34 cell analysis methodology for cells obtained from leukapheresis products mobilized with chemotherapy plus rhG-CSF (r = 0.98), rhG-CSF alone (r = 0.96), and CD34+-selected mobilized leukapheresis products (r = 0.83). A comparison was also made between technicians using both analysis methods. Whereas the correlation coefficient between two technicians using the standard methodology was r = 0.77, the correlation coefficient was much higher when using ProCOUNT (r = 0.99). These data demonstrate that the use of ProCOUNT is associated with less variability between data analyzed by different operators. Also, ProCOUNT is consistent with existing CD34+ cellular analysis methodologies. An additional advantage is the ability to determine the absolute concentration of CD34+ cells, thereby allowing calculation of total CD34+ cell numbers without using WBC counts, which also have inherent errors. The ProCOUNT system provides an automated analysis procedure that minimizes the variables in CD34+ cell analysis and may be useful for standardization of methodology between laboratories.