Evaluation of new automated hematopoietic progenitor cell analysis in the clinical management of peripheral blood stem cell collections.

Peripheral blood stem cell transplantation (PBSCT) is used increasingly to treat patients who have undergone high-dose chemotherapy for hematologic or solid organ malignancies. Successful transplantation and engraftment of stem cells requires the infusion of an adequate number of progenitor cells.1-5 Stem cells are traditionally identified as CD34+ cells by flow cytometry. The minimum threshold value of CD34+ progenitor cells recommended to induce rapid and successful engraftment of hematopoietic recovery is at least 2 × 106 CD34+ cells/kg patient body weight.3-5 Hematopoietic progenitor cells (HPCs) are mobilized from the marrow into the peripheral blood using various regimens and are harvested subsequently by apheresis. Patient responses to stem cell mobilization regimens vary, however, and are influenced by a number of variables, including age, diagnosis, marrow involvement, and preceding chemotherapy.6-10 Thus, determining the optimal time for initiating peripheral blood stem cell collection is often challenging. Historically, the peripheral blood white blood cell (WBC) count has been used as a marker of marrow response to stem cell mobilization, given the convenience of its availability as part of automated complete blood count analysis. However, a growing number of studies confirm that there is little correlation between the peripheral blood WBC count and the number of CD34+ stem cells in circulation.11 Thus, reliance on the WBC count to initiate apheresis may result in inadequate peripheral blood stem cell harvests and the need for an increased number of apheresis procedures. In contrast, peripheral blood CD34 analysis, performed before initiation of apheresis, correlates well with the number of CD34+ cells collected during apheresis.3-5,12-14 CD34 analysis, however, is a labor-intensive and time-consuming laboratory procedure, requiring highly specialized staff. This often creates delays and challenges in patient management. Automated platforms have been developed to identify HPC on Sysmex SE and XE series analyzers.15-20 Analysis is rapid and inexpensive, and performed on the same instruments as are used for complete blood count and automated differential testing. HPCs are detected in the immature myeloid information channel of the analyzers, where all WBCs, except immature myeloid cells, are lysed by the action of surfactants-detergents on the lipid components of the cell membrane. The immature cells are analyzed using radiofrequency and direct current. The radiofrequency signal conveys information regarding cell complexity such as nuclear size and the presence of granules, whereas the direct current signal reflects the size or volume of the cell. Using this technology, moderate correlations between HPC measurements and CD34+ cell counts have been observed.17-19 Although HPC appears to be a useful positive predictor of when to initiate apheresis to obtain desired CD34+ cell yields, HPC levels below predefined cutoffs have not reliably predicted poor CD34+ cell collections. The latter has limited the use of HPC as a surrogate for CD34 analysis in PBSCT. However, strategies for conserving laboratory resources have been proposed, which use HPCs to screen peripheral blood to perform CD34 analysis only on samples with HPC counts below a predetermined cutoff,16,21 thus preventing unsuccessful stem cell harvests while minimizing the risk of missing an adequate stem cell collection. Recently, improved HPC detection (XN-HPC) was developed on a new-generation Sysmex analyzer (Sysmex XN). HPC detection was optimized based on improved sample hemolysis conditions and fluorescent staining. Moreover flow cytometry–based optical detection of XN-HPC was referenced to CD34+ cells.22 Preliminary data from 18 allogeneic and six autologous stem cell donors suggest a good correlation between the new XN-HPC analysis and CD34 analysis by flow cytometry.22 The goal of the present study was to evaluate XN-HPC testing in a larger clinical PBSCT setting. The results demonstrate a strong correlation between XN-HPC and CD34+ cell counts in preharvest peripheral blood and postharvest apheresis products and support the use of XN-HPC as a suitable surrogate for CD34 analysis to determine optimal timing for the collection of stem cells.