INTRODUCTION

The CD34 antigen is expressed on the surface of almost all human hematopoietic stem and progenitor cells. The absolute number of CD34+ hematopoietic stem cells (HSCs) has been shown to correlate with in vitro colony-forming unit (CFU) assay activity and with clinical engraftment in hematopoietic grafts prepared from peripheral blood, bone marrow, and cord blood sources. The single-platform, flow cytometric CD34+ cell enumeration method described here is based on a clinical laboratory protocol, established by the International Society of Hematotherapy and Graft Engineering (ISHAGE), now known as the International Society for Cellular Therapy (ISCT). This protocol can be used to enumerate CD34+ cells in samples of peripheral blood, leukapheresis products, bone marrow, and cord blood. CD34+ cell enumeration by flow cytometry is a rare event analysis, which requires specific gating instructions that are provided in this chapter. Furthermore, the USP CD34+ Cell Enumeration System Suitability RS has been developed to assess the reagents and ensure the correct gating during data acquisition and analysis.