Maybe this could clarify your doubt a bit...

Stem Cell Assay

There are certain cells in the bone marrow whose only function is to produce all the different types of cells in the blood. These cells are called hematopoietic stem cells and when they divide, they produce daughter cells called progenitor cells. Both the stem and progenitor cell groups are equally important in the patient’s recovery process following a stem cell transplant. There are two methods available that can be used to determine the adequacy of the stem cell collection for transplantation. Our laboratory performs both of these assays, the colony-forming unit (CFU) and CD34 analysis.

Colony Forming Unit Assay
The CFU assay involves taking a small portion of the stem cell collection and growing it in a culture. A single stem or progenitor cell in the sample will form a group of mature blood cells and these groups may be counted using a microscope. While the CFU assay is the best indicator of the ability of the stem cells to grow in the patient after transplantation, this testing procedure is not the ideal choice for most laboratories. The major drawback to the CFU test is that the results are not available for 2 weeks. Thus, the CFU test cannot be used to determine on a day to day basis when the patient has collected enough stem cells for transplantation...


http://www.nybloodcenter.org/4e1.htm