What is the optimum number of CD34⁺ peripheral blood stem cells for an autologous transplant?

There is a wide variation in the number of CD34⁺ cells infused for an autologous transplantation. While the minimum number of hematopoietic stem cells for a successful transplantation has been established, there is no definite information on the optimum number or the maximum number of stem cells. It is generally agreed that a minimum dose of 2.5 × 10⁶ CD34⁺ cells is necessary for successful engraftment. Although engraftment of neutrophils and platelets is achieved at that dose, it is delayed and is also negatively influenced by other factors. Reinfusion of 5.0 × 10⁶ CD34⁺ cells results in prompt engraftment of all three cell lines and should be the preferred target. At least one study indicated that a very large number of cells (>15 × 10⁶ CD34 ⁺ cells) resulted in further improvement in engraftment, decreased packed red blood cell and platelet transfusion, shorter hospitalization, and overall decreased costs. In contrast, a few studies showed that reinfusion of a large number of CD34⁺ cells causes fever and engraftment syndrome and may result in prolongation of the inpatient stay. Marrow culture assays performed post-transplant have shown decreased hematopoietic reserve as well as changes in the stromal cells. These changes persist for several years after the transplant. Generally, salvage chemotherapy is poorly tolerated post-transplant due to impaired marrow reserve. This hypothesis has not been validated by systematic clinical studies. It is undetermined if larger doses of stem cells improves the marrow reserve.